Back in the day …
… we used to make all kinds of sacrifices to preview our work. 160×120, quarter res, effects switched off, layers switched off and so on. In recent years, faster processors and more RAM have allowed us to preview more complete comps at higher res, so those sacrifices aren’t as necessary.
Except for one. With really heavy comps, I only preview part of the comp while I’m working. I delay a full-length preview until I expect to go to lunch or something, or when I’ve previewed all the parts and want to see the whole. Previewing the whole, with a heavy comp, is still a bit of a luxury.
And what Dave said.