Massive lag during gameplay v5

[Zero8000]

As the title says, ever since going up to version 5, I have been experiencing a massive amount of lag, or more accurately, slowdown, as it happens even during offline play. In the menus, sometimes it just freezes out of nowhere; music still plays, but the cursor doesn't move for several seconds. During gameplay, all it takes for me is to move my character or camera a little bit, and suddenly, bam: instant slowdown that can last anywhere from 5 to 15 seconds, on average. It's incredibly annoying. For the record, it never happened in any previous version, nor am I using any WADS. I would like some help.