… and keep the site speedy?

I have one thread that has 2000 replies now …

Holy [whatever]!

… 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

You do open the thread, displaying all entries at once? Do you open the thread in the linear or in the nested view?

Is it a case of the script is not designed for that many replies?

I assume this to be caused by the script. Especially, when generating the entries tree (computing which entry is a reply to which entry) for the nested view the script generates a nested structure, comparing one entry to another entry and putting it onto the correct place in the structure. This is the same structure as you see in the thread trees but with more content (all necessary informations to display the whole entries and not only the informations to generate the links).

or is it more a case of a problem at the host server?

You can not eliminate this as a possible cause but IMHO the probability for the script causing this issue is greater.

Tschö, Auge