I have Win 10 x64 Pro and this is on a new PC and a fresh install using UEFI option to install Windows. Initially the PC would start up lighting fast (at the login screen in no more than 10 seconds, maybe less), but it has suddenly become very slow.
It's not the BIOS, as I get the Windows blue logo quite quickly after booting and then the screen moves on to a full plain light blue screen with the circular dot pattern going round and round. This can go on for perhaps 45-60 seconds, maybe more before the login screen finally appears. After that all is fine again.
I tried unplugging all non essential USB connectors but it has not helped. I tried disabling all non essential applications from the Task manager. No noticeable improvement. Where can I start tracking what might be causing Windows to take forever to boot?