I suggest another cause. When it takes a VERY long time to start doing even the simpler things, often that is because some component of the system is failing during the POST process. That is the first thing the system does on power-up: it goes through the system one device at a time and does a quick and simple test of each to see it it responds with a proper answer. If any device does not, it will stop there and keep re-trying for quite a while before giving up. At that point it may quite completely. Or it may proceed to test all the other stuff, then report an error to you. So, look for error messages. If you don;t see any, look in the BIOS Setup screens for signs of failed or missing items, or devices whose info seems really odd.
If you can't spot a troubled device that way, try a one-at-a-time problem detection scheme. Shut down and open the case. Carefully remove ONE device - say, disconnect a HDD or SSD or remove a RAM module. Try to boot. If the system suddenly goes though the POST process easily, and then maybe alerts you that that particular device is missing, then maybe that's the one that was causing all the delays. If not - if the system is still really slow to POST - shut down, replace that device, and proceed to the next. While you're at it, check that all connectors are plugged in solidly.