The above can be a reason probably. However, system maintenance including defragmentation is supposed to be automatic in Win10..(if that's the issue).
With low count of startup entries, I've many times seen Windows 10 take long all of a sudden.
I used to refer to many articles and some tomshardware / tomsguide troubleshooting, but some of the common ones that I had faced too:
fast reboot in Power Options (some Windows update automatically enable it. If you had disabled this, this might have got reenabled. Even otherwise, disable it anyway)
drivers (some windows update including drivers update can cause instability. Run the device manager by right-clicking on the Windows icon and see if any driver shows a yellow exclamatory mark. It's not necessary that corruptions/conflicts will always be reflected there)
some settings changed in AV recently? Verify. I've seen longer booting times with some changes in KIS, for example
There can be more potential causes, however I can recollect only these for now. Why don't you try'em first?
Do you have Restore points enabled? If so, how many do you now have?
Before performing maintenance on my machine, I will create a new restore point, and then delete all the others, then I will defrag the system. I do use windows built in, but I do not rely on the automated "GUI" version which by default only defrags certain file sizes, I defrag the system via CMD where I can fully defrag the drive.
Firstly, please backup any important data before we start troubleshootinng as this is crucial. You can use a USB, External Hard Drive or the Cloud.
Secondly, please could you post your System Specs as we can get an idea of what hardware you are running.
Thirdly, Please could you download and run CrystalDiskInfoPortable
Make sure you download standard edition portable and extract the zip. Then run it and tell us the condition of your hard drive or ssd.
Say either Good, Caution or Bad
Then after that, download and install SeaTools For Windows
Once it is installed, run the Short Drive Self Test and let us know. It will either say pass or fail
These methods will rule out if your hard drive is going bad.