I've done upgrades on 2 laptops recently with exactly the same problem. Windows 8 ran fine, but Windows 10 was absolutely unusable (the newer laptop took about 3 hours from a cold boot to being able to work after logging in). By far the biggest issue was the hard drive. Swapping it from from 5400 rpm disks to SSD/NVMe made a world of difference. On the newer laptop, every time I had to investigate, the problem was Windows Updates doing stuff in the background, but stuff never finished. Slightly less relevant but removing McAfee (installed from Acrobat Reader) helped every time as well.
My plan is to live forever ... so far so good