I for one have not noticed any difference as I usually either use P mode or Funcion mode. If I remember, I will try to see if I notice any difference. Interesting thought.
Knowing electronics, I would be very skeptical of this actually being a thing. Attu mode just doesn't do gps pos holding, which would be a lessor load on the cpu, but what actually happens is the GPS is still active and in use, it's just not doing pos holding. Which CPU wise isn't much of a load. The heat generated is from video processing which is far more intensive than nav data processing.
I've built my own IMU and quads, even an atmel328p (arduino) running at 8mhz can handle Accelerometer, Gyro and GPS data without lag or any heat. But video processing takes a lot more processing power to do.
In my testing I found switching from NTSC to PAL helped the lag. My theory is that PAL is less FPS and smaller resolution. Thus requiring less processing. But still lagged. If the chips can't release their heat into the heatsink, they do the same thing your tablet will do in over heat situation, they slow their clock cycles, which we see as lag in the app.