A slow boot is often the sign of an HDD error. The boot process is having a hard time reading files.
Start by making sure you have a backup of your files.
When booted open a command window - either powershell, command, or terminal by right clicking the start button. Right click whichever is there and run as admin or select the one open as admin.
This will scan the HDD (or SSD) for errors and fix them >> ***chkdsk c: /f*** You will have to reboot for it to run.
You can run it without the /f and it will let you know if it found errors but it won't fix them. with the /f it doesn't tell you about errors. I like to do that.
It could take a long time if there are serious errors or lots of them - do not interupt it. I have had it take hours.
After that runs - Open the cmd prompt again and enter >> ***sfc /scannow*** This will check the system files for corruption and replace them if necessary.
That will also take a while.
After that enter >> ***DISM.exe /Online /Cleanup-image /Restorehealth***
If it still takes a long time to boot, your HDD is likely about to die. Replace it.