When I recently upgraded my 1520 to windows 8.1 cyan upgrade the experience was absolutely terrible.
Icons, text, everything was really small, the phone would lag and hang, warm up, battery eaten up etc. It was soo bad in fact I was looking for a way to downgrade in a few days after upgrading.
What I ended up doing was a hard reset of the phone after backing up my photos, text messages etc. It cleared all the apps out that I had previously installed but that did not get rid of the 8.1 ugrade so I tried it once more and just reinstalled all of my apps which was easy since the app store remembers what apps you previously had and marks them as "owned" I reinstalled them and all the problems went away.
Apparently like all Windows OS, a clean install keeps the problems away, since then I have been enjoying 8.1 upgrade with no more issues.