Hi !
I had the same issues with my 620 from weeks. I tried everything (reboot, reset, remove SD card, etc.).
I installed the GDR3 preview update (and the minors updates released recently). When it was done, juste after the reboot, the 620 send me a message I never seen before : "your SD card seems have a problem, please try to connect it to your computer" (I received this message in french, so this is the english translation I made : the original english message may be different !).
So, I removed the SD card and all screen issues, freezes, slow camera and slow navigation in music seem disappear ! My 620 is back !
I reformat the SD card on my computer and replace it in the 620 and everything seems nice from 24hrs (music and photo fast enough).
So, I think GDR3 deals better with SD card (and in generally, the recent minors updates improves the smoothness). So, GDR3 + reformat SD card may be a solution?
Anyone with the same experience ?