and keep the site speedy?

I have one thread that has 2000 replies now, most of the replies are just text and the entire thread has about 10 images small in size, I do have avatars enabled though

and when I click to open the entire thread it takes about 15-20 seconds before it fully loads

Is it a case of the script is not designed for that many replies? or is it more a case of a problem at the host server?

Thanks