A Mac has built-in defenses against dangerous temps. The system scans thermal sensors attached to various components. The first step if overheating is detected is to down-clock the processor speed. Apparently that can reduce heating. That could affect frame rates.
If that does not reduce temps, the system can do a safety shutdown to save its brains. I've not have one on our newer Macs but on older Macs you received no advisory saying there was a temperature problem. The screen simply went black.
Having lost my first laptop (15" Mid/Late 2007 Macbook Pro) to overheating, I may be a bit touchy on the subject (twitch, twitch).