Am I misunderstanding how PhysX works? I thought its purpose is to offload physical computation from the CPU a Nvidia GPU?
That's what I knew as well.
Also given your specs (I'm assuming it's a laptop) you shouldn't have problems, honestly. I played D3 on my laptop (Core2Duo @2.26 / 4GB / M9600GT) which for today's standards is truly a shit gaming laptop.
Stuttering like that smells like heating issues. If you have stress-testing tools (
AIDA for CPU - you're gonna need a crack;
FurMark for GPU), give it a go and keep an eye on temps (
HWInfo). If you see either the GPU or CPU going too hot, the safe bet is to stop the test. However I would recommend giving it 2-5 seconds more to see if throttling down starts to happen.
Your issue looks a lot to me like performance throttling caused by high loads. This is especially true in laptops which tend to heat up like a girl when she's getting fingered; and given that you have a 550
M I'm assuming you're gaming on a lappy, not a desktop.
The issue here is that particles should not hit your machine so hard. It was indeed true back in the day of Counter-Strike, but that was due to the shoddy programming more than anything else. Nowadays either everything is laggy or it's laggy when there's generally a lot of stuff happening, or it's not laggy. I, personally, haven't experienced particle lag since 2002 or so