4.1 or 4.2?

Hey! Recently my Nexus 7 stopped working. It had an error message, which persisted through factory resets. I then turned it in to repair and got it back today. It turns out they had replaced the motherboard and when I booted it up I discovered it ran Jelly Bean 4.1.

I bought it back in November and actually never used it on 4.1. I have been quite happy with 4.2 and haven't experienced any of the wild lag issues people have been reporting although it never was as smooth as I could have wanted. My question to you is therefore: What actually causes the lag issues on the Nexus 7? Cheap degenerating memory or is it the 4.2 update and should I keep it on 4.1?