That lag shouldn't happen on today's PCs.
Open the .mdl file with a text editor. If it contains legible ASCII code, it isn't
compiled.
You can obtain the nwnmdlcomp compiler
here.
Compiling a model makes very little difference to performance in most cases, but with very large models it can occasionally prevent lag or toolset crashes. This happens so rarely that I wait until I have a problem before compiling the model. I've found this helps with animation sets, and some high-poly creature models (which might also be due to their animation sets).