I just wanted to add my confirmation of this issue to this thread, along with my findings on it after trying to find the exact cause.
Specifically, I'm refering to the OP's problem whereby when dropped successfully, the CBU97s (and 105s) can sometimes cause a PERMANENT FPS drop (NOT the temporary one which all cluster munitions/big explosions always cause).
Further testing has shown that the mode or use of the TGP is irrelevant to whether or not the effect stays: I've tried dropping the '97 in CCIP mode with the TGP off and have still had the effect happen and I've also tried dropping one using the TGP in point track mode and still witnessed a permanent drop in FPS in some cases.
Of course, it's an imprecise science because the permanent drop doesn't always happen with any method of release.
I have noticed however the problem seems to be more likely to happen in busy areas - places with buildings and civilian traffic alongside the intended targets. I've rarely had it happen if it's just on a few random hostile vehicles in a field.
This also affects multiplayer - everyone experiences the huge permanent FPS drop if the explosion effect stays after dropping a CBU-97/105 and usually requires the mission to be reloaded - we've tested using multiple different hosts and with different people dropping the CBU, it's definitely a reproducible problem and independent of any specific computer setup.
Some other things worth mentioning:
I've never had the issue with CBU-87/103 (it uses a different explosions effect anyway, so that's expected)
Ive tried lowering the explosion particle effects in the associated lua, it makes no difference to the FPS drop and would not fix the fact the effect and sound effect are constantly there anyway.
Just because screenshots are nice, here is an example of the effect using TGP point-track mode. Sorry it's a bit crappy, it's hard to fly and get a good screenshot with 1FPS :D