Why do servers in general boot so slow?
If there is 1-2 TByte RAM in them, it could take 10-20-30-40 minutes to cold boot a server..Why is that? Isn't memory extreme fast in average? Why do the "tests"(?) need so much time?
Servers use ECC memory. When powering up, the memory and the ECC bits are in a somewhat random state and reading a previously unwritten address would trigger a memory/ECC fault.
So, the memory has to be initialized to a determinate state, ie. written to - with several hundred GB or even TB this takes a while. On a warm reboot, this isn't necessary and should be skipped by the firmware.
Also, RAID controllers take quite a bit longer to initialize, partly due to the larger number of disks, possibly staggered spin-up, and integrity checks.
Any classic system would boot way slower if it were to perform detailed hardware tests.
To see an example, disable 'fast memory check' in a desktop's BIOS and you'll see how much an actual hardware test takes.
So to answer, this is like it is because for servers hardware integrity is the most important aspect.