Hi Pinchfire.
Yeah, I completely get what you mean. I've played some high-end games on my PC which require nVidia graphics cards and they work fine on high quality on my computer - I have a Quad Core Phenom 1.8 GHz with 4GB RAM and nvidia GeForce graphics card.
It seems that Sinjid plays fairly well at med/high quality but slower on low. I also get lag with other flash games too.
Basically, you're not the only one :/