This sounds like a pretty open, multi-possibility question, but I'm at a loss. I continually experience my mac falling behind by several words or characters. I mainly notice it in chrome and adium, but it isn't necessarily specific to those. When it happens I look at menu meters and notice the following.
4 cores at idle
The hard drive is sitting idle as well.
1,000 MB of ram free at a minimum
network usage under 10KB/s
If I look at the load averages I'm generally fairly low as well. This last time I was around .2 on all three numbers, certainly nothing close to 4.
What else could be causing extreme slowness like this? While I mentioned typing, this is not exclusive to typing, I also notice long delays on simply opening another tab in a web browser. The symptoms might last for a few minutes or more and then completely subside to the point that things feel fast again.
It might be evident based off 4 cores sitting at idle, but looking in activity monitor shows nothing taking much CPU at all, and the biggest ram usage for a single app was around 300 MB this last time a few moments ago.