FRAPS actually restricts frame rate to the recording speed. If you are running at 400 FPS and have FRAPS set to record at 30, as soon as you flip that switch then the MAXIMUM FPS is 30. The fact that you aren't getting 60 is expected.
Now if you weren't running FRAPS you may find yourself having a significant performance decrease during Magmatron's AoE as well. The performance decrease there may not actually be tied to FRAPS but how well your system performs under those conditions in the first place. If you cut your performance in half from say 60 to 30 during that AoE phase on normal, it wouldn't be unexpected to have a similar performance drop from 30 (the cap) to 15 while FRAPS is running.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.