I don't use FL but lowering the sound resolution can be an easy fix but never a good idea as u miss out important parts of ur mix, eq is next to impossible (unless u wanna keep re-exporting in hi res listening back, then tweaking again in low res - can be well frustrating!!)
best upgrade ur pc mate
edit - there are ways to optimise ur pc (cleaning up ur hard drive etc etc) but I did this once, followed this guide in music tech magazine, and I could hardly notice the difference on the effect of my programs performance
really if u have a good processor and a nice chunk or RAM u shouldn't have this issue