I believe that there’s a minor typo on line 63 of post_analyzer.rb that is preventing memoization of mentions in posts:
https://github.com/discourse/discourse/blob/master/app/models/post_analyzer.rb#L63
In particular, the line is missing an ‘s’ and should be @raw_mentions = raw_mentions.
There is no functional impact of this – both post_spec and post_analyzer_spec pass with and without the correction. The net effect is simply that the mentions are recomputed each time the method is called.
As far as I can tell, there are no references to the singular raw_mention variable: egrep -irn 'raw_mention\b' . --include '*.rb' doesn’t come up with any results other than this single line. While I haven’t written any failing tests of the memoization behavior, I think that this is a pretty obvious bug.