Your motherboard may be doing a full RAM check on every POST. There are often option in the bios to disable that.

DDR5 ram has bandwidth of 32,000–64,000 MB/s[1]. Why does it take 20s to check it unless you have a monstrous amount?

[1] https://en.wikipedia.org/wiki/DDR5_SDRAM

I don't know where the POST code is run, maybe not the CPU ? Also, i'd imagine this sort of code to not parallelise the checks because it try very hard to be bug-free, so we are looking at a piece of code scanning the whole RAM linearly, I don't think you reach anything near the max bandwith of your stick of RAM in this scenario. The problem here is that it does that at every boot, it should do it once, you maybe have disable fast-boot options or equivalent in the bios ?

A huge amount of boot time seems to be because BIOS waits several seconds(!) by default for various hardware to get into a stable state after powering up.