SCP with mediavps runs fine on my PC after *major tweaking.* I have a Toshiba laptop with a 2.8 GHz Celeron chip, 736 MB DDR RAM, and an ATI MOBILITY RADEON 7000 IGP. The major bottleneck for me may have been the front-side bus - Toshiba motherboards are crap.
I did several things to try and lessen the lag - first, I disabled the major framerate decreasing options in SCP (i.e. specular, environment maps, etc.) and enabled img2dds, bitmap caching, and vsync disabling. Then, I reflashed my BIOS and set my AGP aperture* to 64 MB (from 32 MB), which helped the performance somewhat. You might have a lot of junk hogging up free memory, as I did, which severely restricts performance. My solution was to restrict start-up and background programs as much as possible. I also disabled a lot of unnecessary Windows services (
XenTweak and
Black Viper's WinXP Optimization Guide are very, very helpful and contain nifty tips/tricks)
I defragged my PC and generally cleaned stuff up that I didn't use. I also increased the virtual memory a whole bunch. Finally, I downloaded optimized Omega drivers for my video card and set specific OpenGL performance benchmarks for use with FSO (via the great ATI Tray Tools program by Ray Adams.)
Final results: The average framerate increased by 20fps, and I got (as a bonus) 70MB of more free memory. Hooray!... although I try not to look at an Orion in-game. Not because I'm intimidated by it or something, but because it slows down the game speed a hella lot.
*increasing the AGP aperture has been significantly
tested and should boost in-game performance. However, the ways by which you increase it may not always involve your BIOS -though they usually do. I, for example, have a ****ty ACPI Toshiba BIOS that doesn't have the option to increase the aperture, so I had to do it thru ATI's SmartGart.
P.S. I would try and optimize your onboard graphics card's performace as much as possible. What make is it again?
Look to
http://www.driverheaven.net for more help.

EDIT: Links fixed