I could start MP at 40 or so frames, which drop to 35 or lower as I approached a town, and then down to 9 to 11 as things started happening.
To fix this, I turn off aliasing, cut everything to "low" in Options, reduced resolution dramatically. Basicly, took the thing down to minimums, and it was choppy when I need vision the most. Tried windowing it, even. Cut out all unneeded processes...
Turns out to be a setting in my NVidia control panel.
In "managed 3D settings", the "multiple display performance" option was selected. I changed it to "single, and now I can play at full res - 1920x1080. I start out at 62 - 64 frames, and drop to 40ish playing. I can get it even lower as the action builds, but it is PLAYABLE now. My card has a 128 bit bus, which I think rally stuffs it up but, without a different card and power supply, I'm not going to find a better answer.
So, those of you with minimal machines, be sure to check your NVIDIA settings in the driver for obvious whack-job selections. Sorry, can't help AMD users.....