Major Kong:

One topic that I've seen confusing a few people is how Nuke handles caching. At its most basic, Nuke is caching results so that it doesn't have to keep recalculating frames, or specific parts of your tree, in an attempt to keep working in your comp nice and snappy. The details can get rather more complicated, however!

I re-read this article last night. Many Nuke artists don't understand how all the cacheing works in Nuke and in turn but get why things slow down. Even if you have read this do it again.