Thank you so much for explaining this, I had no idea what non-imposters were and it turns out I played around with that setting in an attempt to improve my performance and it was set on 4. Now I have increased it to 20 and the problem is resolved! I'm so happy! I've done a bit more experimenting today and yesterday I maxed out the cache and that seems to have improved performance as well. Now majority of avatars are loading in busy sims (it takes 2-5 mins) and I can see animations. SL is running way better than before!
In the SL viewer the only option I can see is "Enable texture compression," is this the same thing? Is it still worth a try even if SL seems to be performing much better now for me?
I had no idea SL viewer got a big update. I have Firestorm but it runs very poorly, nothing loads at all for a long time. Right now though I'm happy with the performance improvements I've noticed in the SL viewer, it is running much more smoothly and actually playable.