There was a test done on 12th gen Intel (12900 variant IIRC, but my memory is hazy). They limited TDP incrementally down to single-digit numbers, the efficiency kept increasing the farther down you go. My takeaway was that there isn't a "normal" or "optimum".
Presumably that's when you hit the minimum voltage the CPU is allowed to run at. There's still a few other gotchas though.
As CPU frequency goes down, it waits less cycles for RAM. Practically it's a bit of a copout since keeping your RAM at high clocks/voltages doesn't effect more CPU TDP but does use more power. It might explain some of the irregularities in testing at least.
