This is my biggest (well, only) complaint about TextMate. I am handling some sql dump files at the moment and TextMate runs out of steam with files bigger than 5 MB. For these bigger files I use Oxygen or Editix. I'd use TextMate for everything if it weren't for this drawback.
Do you happen to have "Check Spelling as You Type" on upon the opening? This may cause CPU drain issue with TM.