After upgrading to IPS4 from 3.x, there is a rebuild that needs to take place, if you have the server resources available, then you can increase this "amount per cycle" from 50 to 200 or so.
applications/core/extensions/core/Queue/RebuildPosts.php line 29
Example improvement:
16Core Machine, 16GB Ram, SSD Drives, went from 50 per cycle, in about 1 second to 200 per cycle in about 2 seconds. I wouldn't go over 200-300 or so.