This is the same reason why the 24 core GpU model M1 Max was the best choice unless you had very specific needs for extra GPU power that could actually take advantage of the apple silicon GPU cores
Presumably due to thermal throttling.
and so the question becomes: how much can cooling interventions help?
Well, to start, of course there is no fan. So, that's quite a place from which to start.
Pointing a fan at the bottom might be nice, but maybe resting it on a big heat sink works better?