OK, HERE'S THE TEST:
Two things to try, they both seem to work the same, but I need more observations----
BASELINE TEST) You should be using the big shockwaves. That means mv_adveffects.vp is in your "C:\Games\FreeSpace2\mediavps" folder. Check your FPS slowdowns during big explosions.
TEST 1) Leave mv_adveffects.vp in place. Unzip this Shockwave3d.tga file
into your "C:\Games\FreeSpace2\mediavps\data\effects\" folder. Test again, you should see improvement as the smaller file overides the larger one.
TEST 2) Remove the Shockwave3d.tga file. Remove the mv_adveffects.vp file. (Store them safely elsewhere please). Unzip THIS FILE
into "C:\Games\FreeSpace2\mediavps" and test again. This file has had the shockwave animations removed as well, as suggested by Wanderer.
(And for me, the biggest help was using DaBrain's optimized beamglows. Here are DaBrain's optimized beam glows
7zipped into a vp file. The original post is HERE
Extract the file and put it in your "C:\Games\FreeSpace2\mediavps" folder and it will really help beam slowdown problems if you have them.)
I'm running with all my Graphics Features turned on.