Hello!
I used to scour these forums and other sites to find out why the game performed so poorly for my somewhat respectable hardware setup. I finally found a solution that may or may not help others, depending on your hardware and OS configurations. So I joined the forum here to share my success with the frame rate issues in this game.
My Setup is as follows;
CPU: i3-2120 @ stock 3.3GHz
OS: Windows XP SP3
RAM: 4GB DDR3
GPU: Nvidia GTX 550 Ti 1GB
DSP: Soundblaster X-Fi Titanium PCI-e
I used to barely manage 15-40 frames per second with all minimum settings (640x480 resolution, everything at either low or off).
Then I decided to try out something I didn't think would of helped at all. I turned on Hyper-threading in my system BIOS and WOW! Now I can play with scene detail at medium, shadows at high, and everything else turned on low at 1680 x 1050 plus 16x Anisotropic Filtering and 4x Anti-aliasing, and get 45-55 fps at 99% of all locations in the game.
Hyper-threading hurts the performance of many of my other games so it's a pain in the butt to switch it on and off whenever I want to play something other than Saints Row. But I can't argue with the HUGE performance difference.
I hope this helps somebody.