In VLC, Go to Preferences, then the "Input & Codecs" Tab, you should see a Option Called "Skip H.264 in-loop deblocking filter"
It is a enhancement for low-quality videos, but it may lag in HD Videos.
Change it to "All" Then it shouldn't lag anymore.
I find it wierd that you have LAG with HD Videos since My laptop with a Core Duo and GMA 950 doesn't lag with 1080p videos...
Hope it helps!