We have the same amount of RAM allocated, so the difference in versions shouldn't matter. I'm on Mame UI64 0.146u1 (from about June 2012) Now I can't imagine that mine being a 64 bit version, and yours being 32 bit should have anything to do with your performance. So let's talk about your Mame install, and what version of the game you are using.
So you've got something wrong in your setup. On Killer Instinct 2, once the game initially loads, there is no lag, slowdown, sound glitches, nothing. I'm 'streaming' my mame games over a wired network from my file server to my HTPC. My video card is a slim form factor GTX 520. (This is a dual core, to your Quad core.) I have the same amount of RAM as you, and a slow 5200 RPM hard drive. The setup that I'm using for my HTPC is an old AMD Athlon X2 2.4GHZ.
Alright Redd, I finally got around to looking into this.
