Weird "invisible" thermal throttling at ~80°C?

[Fierelier]

Hello forums,
I have both a T420 (Intel graphics) and X201 that throttle somewhat inconsistently at high temperatures. The weird thing about this throttling is that it happens before T-junction, and when I look with hwinfo, there are no signs of throttling. The CPU throttling sensor says it's not throttling, and the CPU clocks are not reduced, but the system feels very much slower.

I think it might be related to the board temperature sensor (one of Lenovo's) reaching 80°C, but I'm not certain, the board temperature is quite close to the CPU's.

Does anyone have a clue what this is about? I tried searching for it when it occured for my T420 in summer, but I couldn't really find much info on this specific type of throttling.