Why does my computer turn on so slowly?

There are several explanations.

The hardware obviously plays a role.
Your computer may not perform “naturally”. The components that make it up make it not fast, the speed at which it processes data is low. If it is a fixed one, you can eventually change some components. It seems to me that it is the processor and the RAM that are involved in this work.
SSDs are known to be fast as well.

There is a software part.
The more programs you have running at startup, the longer it takes for the computer to turn on. Don’t hesitate to prohibit programs from launching at startup if they are not needed right from the start (a word processor doesn’t need to load at startup, it can wait to be requested without any problem for example).
For Windows 10, just search for “startup” and the computer will suggest “startup applications” which will open a settings page. There you can disable unnecessary programs. Be careful not to disable all those you do not know, some are necessary (mouse, keyboard, sound, graphics etc.). Only deactivate programs that you know and are sure that they will not create problems if they are deactivated.

Memory can also affect boot speed.
The fuller the computer’s hard drive is, the longer it may take to boot up and launch applications, especially if it is not defragmented regularly. The computer will look for information where it is on its disk(s), so if it needs some information on one side and the rest on another, it will take a little time to go and find it.

Malware can be a factor.
Some viruses or Trojan horses are known to slow down computers. So always keep an active anti-virus program up to date. Remember to scan regularly and be careful when downloading anything.

Heat.
Computers are slower when they are hot! The components should not exceed a certain temperature. So when it’s hot, the computer is deliberately putting less strain on its components to avoid overheating. Think of dusting it once a year at least at the level of the fans (if it is a fixed one, you can even open it to dust delicately inside). There are dedicated dry compressed air cans. If you have a laptop that heats up, there are fans created specifically for them.
Also check with professionals to change the thermal paste regularly.

It is probably a machine running Windows.

At the time I was at work, the norm was to reinstall Windows every year, because the performance was going down with time. But I admit that it was not yet Windows 10.

Try to see if there are any Linux groups in your city/neighborhood/school, and try the performance of your machine by booting on a Linux USB key.