bad mission choice. I dont think everyone has Technical Superiority installed. I suggest Clash of the Titans II as a test mission, as it is a campaign mission.
Also, you need the right builds and command lines.
first 32_bit build: it should be the _32-bit build, it supports tgas and jpgs as well as HT&L. HT&L should have serious flickering issues and the lighting should be rather flickery too. The thrusters will _not_ be displayed through the ships though.
Command lines: -htl (duh.) (-t32 -32bit older builds) (-jpgtga newer builds) -fps
Clash of the Titans II:
max FPS: 70 / 47 / 70 / 62
min FPS: 63 / 20 / 49 / 33
avg FPS: 69 / 27 / 58 / 41
edit: i'll test some more since the drain seems to be sporadic and does not happen every time.
-edit2: or better yet, i'll make a test mission to cover various slow down issues.