Sry, I have misread that.
I’ve now measured the time difference on our realistic testsystem between a manual reindex action (took ~3min30sec), and restaring the xwiki container with solr.synchronizeAtStartup=true (took ~4min 45sec)
Sooooo, I’m not quite convinced 
. Maybe there is a bug somewhere.
Since I’m currently evaluating whether or not we use an external SOLR docker container instead of the embedded way, this issue has no priority for us. (Will be interesting to see what happens in that setup)
Thank your for your feedback nevertheless.
BR