I've noticed it a lot too. Today when I was typing I noticed it, (I was in a very crappy connection area) then I heard the chime of a new email and the lag time went away.
If I had to guess it's associated with trying to get some information over Edge.
ive noticed it occassionally. Also it "seems" to happen most when im typing wiht two fingers. I say seems because i noticed that I was typing so fast I would often hit the touchscreen with both fingers at once, causing the iphone to ignore the simultaneous tap altogether. Maybe that is what some of you are experiencing.