Page load performance / Page Cache relevant?

Exactly. Happy to provide more details / logs if you tell me what exactly would help (e.g. output of a certain XWiki logger, full thread dump etc.). We even disabled all side panels.

xwiki.cfg:
xwiki.authentication.authclass=org.xwiki.contrib.oidc.auth.OIDCAuthServiceImpl

xwiki.properties:
nothing

page with object of class XWiki.OIDC.ClientConfigurationClass:
This is the xml of the exported object (I just removed Google client id and secret):

  <object>
    <name>GoogleSSOConfig</name>
    <number>0</number>
    <className>XWiki.OIDC.ClientConfigurationClass</className>
    <guid>680745ea-d673-4bc2-b408-a7bf1f58920c</guid>
    <class>
      <name>XWiki.OIDC.ClientConfigurationClass</name>
      <customClass/>
      <customMapping/>
      <defaultViewSheet/>
      <defaultEditSheet/>
      <defaultWeb/>
      <nameField/>
      <validationScript/>
      <authorizationEndpoint>
        <disabled>0</disabled>
        <name>authorizationEndpoint</name>
        <number>7</number>
        <prettyName>Authorization OIDC endpoint</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </authorizationEndpoint>
      <clientId>
        <disabled>0</disabled>
        <name>clientId</name>
        <number>11</number>
        <prettyName>Client ID</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </clientId>
      <clientSecret>
        <disabled>0</disabled>
        <name>clientSecret</name>
        <number>12</number>
        <prettyName>Secret</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </clientSecret>
      <configurationName>
        <disabled>0</disabled>
        <name>configurationName</name>
        <number>1</number>
        <prettyName>Configuration name</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </configurationName>
      <groupsClaim>
        <disabled>0</disabled>
        <name>groupsClaim</name>
        <number>2</number>
        <prettyName>Group claim</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </groupsClaim>
      <idTokenClaims>
        <cache>0</cache>
        <disabled>0</disabled>
        <displayType>input</displayType>
        <freeText>forbidden</freeText>
        <largeStorage>0</largeStorage>
        <multiSelect>1</multiSelect>
        <name>idTokenClaims</name>
        <number>20</number>
        <prettyName>ID Token Claims</prettyName>
        <relationalStorage>0</relationalStorage>
        <separator>|</separator>
        <separators>|,</separators>
        <size>5</size>
        <unmodifiable>0</unmodifiable>
        <values/>
        <classType>com.xpn.xwiki.objects.classes.StaticListClass</classType>
      </idTokenClaims>
      <logoutEndpoint>
        <disabled>0</disabled>
        <name>logoutEndpoint</name>
        <number>10</number>
        <prettyName>Logout OIDC endpoint</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </logoutEndpoint>
      <logoutEndpointMethod>
        <disabled>0</disabled>
        <name>logoutEndpointMethod</name>
        <number>16</number>
        <prettyName>Logout endpoint method</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </logoutEndpointMethod>
      <logoutMechanism>
        <disabled>0</disabled>
        <name>logoutMechanism</name>
        <number>17</number>
        <prettyName>Logout mechanism</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </logoutMechanism>
      <scope>
        <cache>0</cache>
        <disabled>0</disabled>
        <displayType>input</displayType>
        <freeText>forbidden</freeText>
        <largeStorage>0</largeStorage>
        <multiSelect>1</multiSelect>
        <name>scope</name>
        <number>19</number>
        <prettyName>Scope</prettyName>
        <relationalStorage>0</relationalStorage>
        <separator>|</separator>
        <separators>|,</separators>
        <size>5</size>
        <unmodifiable>0</unmodifiable>
        <values/>
        <classType>com.xpn.xwiki.objects.classes.StaticListClass</classType>
      </scope>
      <skipped>
        <disabled>0</disabled>
        <displayFormType>select</displayFormType>
        <displayType>select</displayType>
        <name>skipped</name>
        <number>18</number>
        <prettyName>Is authentication skipped ?</prettyName>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.BooleanClass</classType>
      </skipped>
      <tokenEndpoint>
        <disabled>0</disabled>
        <name>tokenEndpoint</name>
        <number>8</number>
        <prettyName>Token OIDC endpoint</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </tokenEndpoint>
      <tokenEndpointMethod>
        <disabled>0</disabled>
        <name>tokenEndpointMethod</name>
        <number>13</number>
        <prettyName>Token endpoint authentication method</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </tokenEndpointMethod>
      <userInfoClaims>
        <cache>0</cache>
        <disabled>0</disabled>
        <displayType>input</displayType>
        <freeText>forbidden</freeText>
        <largeStorage>0</largeStorage>
        <multiSelect>1</multiSelect>
        <name>userInfoClaims</name>
        <number>21</number>
        <prettyName>User info Claims</prettyName>
        <relationalStorage>0</relationalStorage>
        <separator>|</separator>
        <separators>|,</separators>
        <size>5</size>
        <unmodifiable>0</unmodifiable>
        <values/>
        <classType>com.xpn.xwiki.objects.classes.StaticListClass</classType>
      </userInfoClaims>
      <userInfoEndpoint>
        <disabled>0</disabled>
        <name>userInfoEndpoint</name>
        <number>9</number>
        <prettyName>User info OIDC endpoint</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </userInfoEndpoint>
      <userInfoEndpointHeaders>
        <cache>0</cache>
        <disabled>0</disabled>
        <displayType>input</displayType>
        <freeText>forbidden</freeText>
        <largeStorage>0</largeStorage>
        <multiSelect>1</multiSelect>
        <name>userInfoEndpointHeaders</name>
        <number>15</number>
        <prettyName>User info endpoint headers</prettyName>
        <relationalStorage>0</relationalStorage>
        <separator>|</separator>
        <separators>|,</separators>
        <size>5</size>
        <unmodifiable>0</unmodifiable>
        <values/>
        <classType>com.xpn.xwiki.objects.classes.StaticListClass</classType>
      </userInfoEndpointHeaders>
      <userInfoEndpointMethod>
        <disabled>0</disabled>
        <name>userInfoEndpointMethod</name>
        <number>14</number>
        <prettyName>User information endpoint method</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </userInfoEndpointMethod>
      <userInfoRefreshRate>
        <disabled>0</disabled>
        <name>userInfoRefreshRate</name>
        <number>22</number>
        <numberType>integer</numberType>
        <prettyName>User info refresh rate</prettyName>
        <size>5</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.NumberClass</classType>
      </userInfoRefreshRate>
      <userMapping>
        <cache>0</cache>
        <disabled>0</disabled>
        <displayType>input</displayType>
        <freeText>forbidden</freeText>
        <largeStorage>0</largeStorage>
        <multiSelect>1</multiSelect>
        <name>userMapping</name>
        <number>5</number>
        <prettyName>User mapping</prettyName>
        <relationalStorage>0</relationalStorage>
        <separator>|</separator>
        <separators>|,</separators>
        <size>5</size>
        <unmodifiable>0</unmodifiable>
        <values/>
        <classType>com.xpn.xwiki.objects.classes.StaticListClass</classType>
      </userMapping>
      <userNameFormatter>
        <disabled>0</disabled>
        <name>userNameFormatter</name>
        <number>4</number>
        <prettyName>XWiki username formatter</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </userNameFormatter>
      <userSubjectFormatter>
        <disabled>0</disabled>
        <name>userSubjectFormatter</name>
        <number>3</number>
        <prettyName>Subject formatter</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </userSubjectFormatter>
      <xwikiProvider>
        <disabled>0</disabled>
        <name>xwikiProvider</name>
        <number>6</number>
        <prettyName>XWiki provider</prettyName>
        <size>255</size>
        <unmodifiable>0</unmodifiable>
        <classType>com.xpn.xwiki.objects.classes.StringClass</classType>
      </xwikiProvider>
    </class>
    <property>
      <authorizationEndpoint>https://accounts.google.com/o/oauth2/v2/auth</authorizationEndpoint>
    </property>
    <property>
      <clientId>xxx</clientId>
    </property>
    <property>
      <clientSecret>yyy</clientSecret>
    </property>
    <property>
      <configurationName>default</configurationName>
    </property>
    <property>
      <groupsClaim></groupsClaim>
    </property>
    <property>
      <idTokenClaims/>
    </property>
    <property>
      <logoutEndpoint>https://oauth2.googleapis.com/revoke</logoutEndpoint>
    </property>
    <property>
      <logoutEndpointMethod></logoutEndpointMethod>
    </property>
    <property>
      <logoutMechanism></logoutMechanism>
    </property>
    <property>
      <scope>
        <value>openid</value>
        <value>profile</value>
        <value>email</value>
      </scope>
    </property>
    <property>
      <skipped>0</skipped>
    </property>
    <property>
      <tokenEndpoint>https://oauth2.googleapis.com/token</tokenEndpoint>
    </property>
    <property>
      <tokenEndpointMethod></tokenEndpointMethod>
    </property>
    <property>
      <userInfoClaims/>
    </property>
    <property>
      <userInfoEndpoint>https://openidconnect.googleapis.com/v1/userinfo</userInfoEndpoint>
    </property>
    <property>
      <userInfoEndpointHeaders/>
    </property>
    <property>
      <userInfoEndpointMethod></userInfoEndpointMethod>
    </property>
    <property>
      <userInfoRefreshRate/>
    </property>
    <property>
      <userMapping/>
    </property>
    <property>
      <userNameFormatter>${oidc.user.email}</userNameFormatter>
    </property>
    <property>
      <userSubjectFormatter>${oidc.user.email}</userSubjectFormatter>
    </property>
    <property>
      <xwikiProvider></xwikiProvider>
    </property>
  </object>

This config works.

What did not work, as said, was to remove that xwiki page and instead insert the following into xwiki.properties:

oidc.endpoint.authorization=https://accounts.google.com/o/oauth2/v2/auth
oidc.clientid=xxx
oidc.secret=yyy
oidc.groups.claim=
oidc.idtokenclaims=
oidc.endpoint.logout=https://oauth2.googleapis.com/revoke
oidc.scope=openid,profile,email
oidc.endpoint.token=https://oauth2.googleapis.com/token
oidc.userinfoclaims=
oidc.endpoint.userinfo=https://openidconnect.googleapis.com/v1/userinfo
oidc.user.mapping=
oidc.user.nameFormater=${oidc.user.email}
oidc.user.subjectFormater=${oidc.user.email}

It would then send the custom claims, although they are set to NULL, filled in a rather meaningless way like {"id_token":{"":null},"userinfo":{"":null}}.
The parameters are the same as in the page object so the treatment must be different at some point. We also tried completely leaving out these claim params (like oidc.userinfoclaims) from the xwiki.properties, which did not change the result.