See here:
https://forum-en.msi.com/index.php?threads/where-is-the-ia-cep-option-in-the-bios.383312/
On the B660/B760 boards, the "IA CEP Support" option seems to be missing/ineffective if combined with certain CPU models. So as soon as you seriously try to undervolt the CPU, "IA CEP" will always intervene and limit the performance as well (apparently through a process called clock-stretching, a form of throttling).
On an MSI MAG B660M MORTAR DDR4 + i5-12400 (with H0 stepping), there is no IA CEP option available in the BIOS. The 12400 being used is the H0 variant, which - according to the table in the thread i linked - cannot be undervolted without a performance drop, neither on Z- nor on B-chipset boards. And my results confirm that. Luckily, the power draw of the 12400 is natively quite low, so it's no big deal there. With a 13600KF there would be more reason to lower the power draw.
Just a few Cinebench R23 Multi results to see the effect of IA CEP intervening:
CPU Lite Load 12 (default) - CB23 score: 12065 - total system power draw: 124 W.
CPU Lite Load 9 - CB23 score: 11747 - total system power draw: 119 W.
CPU Lite Load 6 - CB23 score: 5493 (!) - total system power draw: 94 W.
Undervolt -0.05V - CB23 score: 5900 (!) - total system power draw: 99 W.
So there is no sense in undervolting significantly, the performance drops way more drastically than the power draw at some point.
In a case such as this, you have two choices:
1) The easy method: You find out the lowest CPU Lite Load mode that allows you to keep roughly the full performance (meaning, IA CEP doesn't yet intervene). Testing with two i5-12400, on a B660 and on a Z690 board, i found that to be Mode 9. The default on the Z690 board with the latest BIOS version was Mode 16 (!), so Mode 9 was a good improvement already. On the Z690 board i have roughly the same CB23 score (12200-something points, there's always a bit of variance), at 10W lower power draw and 4°C lower temperatures compared to the default Mode 16, pretty nice for such a frugal CPU model and not being able to lower the mode to the usual degree.
2) The difficult method: You'd have to find some way of combining different undervolting methods that keep IA CEP from becoming active, while still achieving a lower voltage than CPU Lite Load alone (otherwise it wouldn't be worth the hassle). But this then really gets a bit complicated.
