The long restart duration could be caused by a depleted entroy pool: Errors and Warnings in Logfile - #8 by frsc
Apparently, Xwiki should use /dev/urandom, but something in it somehow seems to read from /dev/random, causing the starting server to block for a long time in case of insufficient entropy.
A “find / > /dev/null” on the server does wonders in this case (and can be interrupted / killed once the server started successfully).