Earlier quoted context omitted.
Great question. I'm wondering the same thing. I also built a PC in 2017 and installed Windows 10 on it. It initially had a 7s boot time following bios startup. Several updates later, the boot time is now close to a minute and app startup times are noticeably slower as well. I've always been very careful about what software I install on it. I've done all of the standard troubleshooting, but so far it hasn't gotten bad…
Task Manager → "Start up" tab Disable everything and see if it boots faster.
Handy hint, it has an option to hide all the default Microsoft stuff so you just see third party.