XQuartz startup very slow
Some software creates lock files to indicate to other pieces of software that a resource is being used. In theory they will delete the lock file once they are done using the resource. However if they crash they can end up leaving the lock file behind.

This is what is happening here. XQuartz is launching and wants to update the font cache but it sees the font_cache.lock file so thinks the font cache is already being updated. Most likely it isn't and it just crashed the last time it ran, leaving the lock file behind.

Just delete the /opt/X11/var/run/font_cache.lock file and try XQuartz again.