In particular a place I used to work had a plugin for threaded comments in Jira. The specific one we were using slowed things down noticeably with the DB on the same server, but not too much to be an improvement in overall usefulness.

Then we decided trying to make our Jira more reliable by splitting the DB out into a separate clustered DB system in the same data center. The latency difference going through a couple of switches and to another system really added up with those extra 1600 or so DB calls per page load.

We ended up doing an emergency reversion to an on-host DB. Later, we figured out what was causing that many queries.