There seems to be a significant performance bottleneck in Forums with large numbers of posts and users. This appears to be due at least in part to the ranking of each user every time a new action is taken. In our (NYU’s) instance this query turned out to be our #1 DB time consuming query:
The code looks to the correct indexes, and we measured that the query is taking about 10 milliseconds to complete (maybe more like 30 milliseconds once you factor in network latency). However, because this query runs once for each user, this time adds up significantly in sites with many users and a large number of Topics/posts.
Performance could be improved by adding an option that allows the admin to turn off ranking to or otherwise change the code to calculate user ranks in bulk.
Link to image of all page requests in Firebug: https://drive.google.com/file/d/0B_Eit0JDnaT1dk03bEFMdXkxQ2s/view?usp=sharing