Another "unread replies" complaint from me too. It's usually the first thing that lags. It's annoying because "unread posts" and "unread replies" are the only way to use this forum effectively.
I know this is months old, but I'm still having the same issue. It's to the point I usually abandon the unread replies and just browse around sub forums. I usually get several sub forums loaded up before the unread replies will load...if it loads. It seems to hang indefinitely about half the time.
Yeah, it's been getting pretty bad. The really stupid part of it all is the speed at which the unread replies thread will load up, but will flag any replied to thread with a smiley page icon on the left. So, tagging threads we have commented in with a custom icon when pulling up several orders of magnitude more data than the read replies page lists does nothing to slow it down, but only trying to pull up those threads causes timeout errors?
For funsies I tried to look at the SMF code that generates the "unread replies" page. Noped out of there real quick.
Sounds about right from what I've seen of other chunks of SMF code.
I love FOSS software, I've got a warm spot in my heart for SMF specifically, and I know migrating between forum platforms is a colossal PITA... but I'm not sure SMF was ever meant to handle the loads we're subjecting it to as this is hands down the largest SMF community I've seen. Search has been mostly broken for years now. There's been multiple back-end hardware upgrades necessitated. We're having problems with basic functions that larger communities don't have problems with. I'm not convinced throwing more hardware resources at this will fix our problems at this point.
There's no shame in admitting that the community has been so successful that it's outgrown it's initial forum platform... but it does present its own unique set of issues if proven true.