I have a some sort AMD with 1GB SD ram, ATI 9250 AGP, haven't really have the time to test W10 on it, but previous attemp with insider build wasn't going so well as it seems to have some issue with graphic where the animations bog down the system. Well, see when I got time to play around with it.

...

Anyway, no "heavy" problem on my main machine (i5 Sandy Bridge) with SSD. 20 sec boot.
Nah, even my several years old W7 don't runs "heavy" on SSD anyway.
If you're still on traditional HDD, check for health status with Crystal Disk Info, see if your HDD acting up, bad sector, reading error, etc.

And yes I also recommend clean install at the end.
After upgrade, fetch your free key, format, re-install with ISO. Yea it's kinda annoying process but... for the sake of a clean stable system.
I bet large majority of the problems people talked on the Internet usually comes from the "upgrade", either inherited problem or problems created after migration.