What CPU you have with that B660 board?
VRM (voltage regulator modules) are around the CPU socket, usually under some heatsink. Theses modules take 12V and convert it into what the CPU needs (0.6~1.5V) to operate.
Each board has many VRmodules. They split the CPU current (Ampere) into multiple phases. Some boards have high capacity and high quality modules and some not. More expensive VRM are producing less heat because of higher (conversion) efficiency, or some boards have too many of them so each VRM is not stressed much.
Usually VRM have maximum operating limit up to 115~125C but some of them start to throttle current even from 85~90C and resulting to lower CPU performance.
In order for your board's VRM to get to 90C could mean
1. The board has too few of them
2. VRM are low quality
3. The CPU is drawing too much current for the board voltage supply system
4. You dont have enough air flow around the CPU socket.
One or more from the above can be true.
EDIT:
After a little research it turns out that ASUS Prime B660 Plus D4 is one of the lowest quality B660 with only 6-phase VRM and probably each of them is kinda weak.
Also the VRM heatsink is very basic (small) I would say.
Here is an operating temperature demonstration with a 165W CPU
VRMs are over 100C and CPU performance is throttled
View attachment 12627
Here is the same load but with a dedicated fan blowing air directly on the VRM's heatsink
CPU performance is now on par with the rest of B660 boards
View attachment 12628
The best you can do is to take a fan (preferably a 120mm) and place it, maybe on the back of the GPU in a way that blows air around the CPU socket.
You also have to consider that warm/hot air must be able to leave the case. So consider to not place that fan against the general air flow (usually from front/bottom to back/top)
Here is the link of the B660 board review I got the pics from