Kerbal Space Program

Reproduce:
- Create a craft that can go fast;
- Go fast™;
- Watch framerate get cut in half or more as supersonic/heat FX are drawn;

Tested on this machine: 4GHz i5 4670k, 16GB RAM, GTX980.
Tested with following min. settings: No mods (except for LinuxGuruGamer's ShowFPS), 1280x720 resolution, everything off or turned to minimum, just with 'normal' set AeroFX quality, 'default' FPS limit, no vSync.
Tested with following max. settings: No mods (except for LinuxGuruGamer's ShowFPS), 2560x1440 resolution, everything on or turned to max, '120' FPS limit, sync every vBlank.

Results min settings:
~30 part fighterjet: 260 fps AeroFX below 'normal'. 130 fps AeroFX at 'normal'.
~150 part Mk3 SSTO: 90 fps AeroFX below 'normal'. 30 fps AeroFX at 'normal'.

Results max settings:
150 part Mk3 SSTO: 60 fps AeroFX below 'normal'. 20 fps AeroFX at 'normal', sometimes dipping to 10-12 fps when going fast™ at around 20k altitude. (yes, sometimes, sorry, was unable to reproduce that insane drop reliably)

Kind of a big deal in a game where heat and atmospheric effects play a big role in my opinion. Especially since my machine is far from slow, and I can't imagine what it's like for people with far worse PCs.