I have an Digicom DIGIS-386L with a 486DLC-33 CPU. I noticed a strange behavior with boot speed

For example 3dbench 1.0

With keyboard connected during power-on, my DLC33 gives 18.5 fps.
If I start the system without keyboard, and plug it in just before memory count, the same CPU gives 20.5 fps.

Cache L1 and L2 are enabled in BIOS, all wait states are 0, so it seems not a CPU or crystal problem.

I suspect this is related to the 8042 keyboard controller affecting POST paths, which somehow changes chipset initialization and timings. There is no option for fast A20 in BIOS

Does anyone know:

Why keyboard presence at POST affects 486DLC / ISA performance?
How to make the system always run at “fast mode” regardless of keyboard?
Any BIOS / register tweaks for this board to stabilize speed?

Thanks in advance for any tips.