Disclaimer: this fix does not apply to everybody ;)
I've had several Android devices since Android 2.1 and I've always been bothered with that occasional lag you get way to often!
I've tried many ways to squeeze that tiny little bit of speed from the hardware, with little to no success. Sometimes it would be a bit better yet not perfect; Sometimes the 'better' was just placebo.
My current phone is a galaxy nexus. I'm generally happy with it but it felt laggy. From time to time I would pick up the nexus of a friend and feel it was much more smooth and responsive.
Well, I finally found the reason: my wallpaper.
I am a hobbyist photographer. When I started this hobby I took an oath: "I will never have a wallpaper that I didn't shoot myself.". A good way to keep the motivation to use this awfully expensive gear :)
Now my mistake was to not resize the pictures and use them directly as wallpapers. I'm talking about 18ish MP high quality pictures. Once resized to the desktop size (that's about twice as wide as the device held in portrait mode) lags are gone.
When I discovered this, my phone was running Android 4.1.2 I believe, it is now running Android 4.2.1. Maybe it no longer applies to 4.2, but lags didn't come back.
This is definitively not placebo :)