Thanks for the response. I have beat my head against a wall for months trying to get to the bottom of this. I already tried and abandoned using multiple threads within drawRect:, and I agree there is a global lock :). For the case I benchmark there are no bitmaps besides NSFillRect on the background color (which is surprisingly slow, but unavoidable without doing something weird like drawing background colors on a separate layer).

I've confirmed that I always use the same NSFont in the attributed string used to create the CTLineRef.

If you're curious, the text drawing code is in drawTextOnlyAttributedStringWithoutUnderline:atPoint:positions:backgroundColor:graphicsContext:smear: and is located here: https://github.com/gnachman/iTerm2/blob/master/sources/iTerm...

If you're able to spare some cycles, please get in touch with me. [email protected].