Mine has been fine since I had it back when it was released, maybe your running out of space on the internal memory? Any background tasks stuck running in task manager?
What you can try is right clicking on the C drive and then properties. Select Disc Clean-up, let it do its thing then select Clean up system files. Again let it do its thing and that should free up a bit more space for you.
If not then there must be something that's taking up some space. The...
I have just sent mine in for this problem as well as a couple of others.. over heating and crappy battery life. Should get it back in a week or two. The 920 is about 11 months old.
No stuttering or lagging with mine, games I play are GTA, Asphalt 8 and Halo. The 2520 may have a lower rest camera but its by no means a bad front camera, I have had no complaints from people about poor quality when I Skype.