We noticed that Remex made out test suit slower (for me 7->10s, for BD, 2-3 times slower).
https://gerrit.wikimedia.org/r/c/mediawiki/extensions/DiscussionTools/+/606002/
This task is to investigate if this is an issue, and if there are any parameters we can tweak to make it faster.
I put some timing logging in CommentTestCase::createDocument, and for the test case that uses cases/en-big-oldparser/en-big-oldparser.html it took 150ms, vs 4ms when using loadXML. The loadXML code can be recovered by reverting CommentTestCase to the previous revision.
After disabling xdebug, Remex speeds up, but it is still slower. It is 7-10x slower instead of 30-40x slower.
The absolute times aren't terrible: about 40ms for a 30-section talk page (4ms with loadXML) and 250ms for a very large 180-section archived talk page (30ms with loadXML).
The performance of the CommentParser is meeting the performance expectations we have. As such, marking this task as resolved.