There's no perfect answer to this question. We have found that about 5000 replies is where the effects of us operating on a big thread have a noticeable impact on overall site response.
Not sure exactly what is happening, but when a moderator opens a thread, even just to read it, we have a bigger impact on the thread than a typical user does.
So, even if we are not attempting to change anything, we slow FR down for everyone.
If you would create a new thread at about 5000 replies and contact us via abuse report, we can lock the current thread and force replies to the new one.
Thanks