Slow Startup Resolved by Ctrl Alt Delete : what am I doing?

Can anyone explain what I am doing? I free-upgraded to W10 over W7, and initially it seemed ok, but computer became increasingly sluggish across the board (particularly browsing, but also starting programs) until it had been running for about 60 minutes. I followed some online tips like reducing page size, and then the slowness seemed to spread to the start up - the little rotating dots virtually froze - and windows didn't like closing or sleeping either. In desperation I tried Ctrl Alt Delete while the dots were frozen on start up. Windows seemed to reboot and the slowness problem seemed to be resolved. Is this just a coincidence or can anyone explain what is happening?
thanks Joe

First off....did you check to make sure your PC (desktop/laptop) had windows 10 support (driver support) before upgrading it. Just because you have a qualifying OS to upgrade doesn't mean your hardware is. I have 2 Sony laptops that have 7 Pro, which qualifies but there are no drivers for them. So I have to keep 7 on them.

This is a common occurrence with the upgrade route. A clean install is the only way to fix that. Supposedly, after the upgrade is in place, you can do a clean install of 10 using your 7 product key. It's supposed to be converted to a 10 key during the upgrade. I've not done one as all mine are clean installs, so I can't confirm this.

You can make a system back up (system image).....keep that on an external source and try the clean install route and if that doesn't work use the system image to revert back.

Hi; thanks for replying. How would I know whether I had driver support? (this was never mentioned; windows upgrade appeared to endorse my hardware). I am running on Intel i7 Q820 @ 1.73 Ghz, and it's a pretty fast 64 bit Dell Studio XPS 1645 with SSD; maybe 4/5 years old. The whole show seemed to work fine for a month. I know it sounds like magic, but since I pressed Alt Ctrl Delete during one particularly slow start up, everything has speeded up. Do you know what pressing Alt Ctrl Delete does during startup? I don't recall it having any effect at all previously (I was just desperate to break out), but recently it seems to reboot (or maybe it just looks like a reboot); thanks, Joe