I've noticed the same lag. It seems this happens every six months or so on here, doesn't it?

Putting something aside for a while works very well, especially if you get frustrated by something you are writing. I know that when I get frustrated with something (like this query letter), it will take me ten times longer to get through one sentence then when I am more relaxed and patient with it. So definitely put it aside for a few days, get your head off of it, do some other stuff, and then take it out again. :)