Typically, slowdowns are related to the parser having to process really long lines, and LaTeX tends to have really long lines. Perhaps that is what you are seeing?
Hmmh, you seem to be right, although really long in my case means 52 words, or 353 bytes. I don't think that TextMate was that sensitive when my system was still Tiger, at least it never really bugged me. But now I can hardly use it.
Nicolas Schmidt