We have 5M posts and searching is pretty snappy. Hosting on a 6 shared vcores of a 2.8Ghz Xeon E5-2680 with 16GB RAM and SSD storage. That said I don’t have metrics on how many simultaneous posts our users initiate, there might be locking issues.
Posting, on the other hand, is often quite slow. It can take 6-7 seconds for a post to go through. Discourse is non-blocking when it chugs away so that doesn’t kill the user experience, though.