Go ahead and read it -- even take notes -- but then look at them and organize them.
Start with the big issues. Don't say change "this grammatical expression" to "that one" for a page and then get around to mentioning that the first half and the second half have a fatal discontinuity of tone -- not just the language and attitude, but the events don't mesh into one story. Or that the heroine's as flat as a pancake except for a couple of unconvincing incongruous acts.
For one thing, if the writer goes through the story to make them congruous, odds are ninety-nine percent of your changes will be eliminated in the re-write without the writer's even trying. Yes, he may introduce all sorts of errors of that ilk in the next draft, but not that particular one. (Which is why if you see a systemic error, you should not go "change this to that," "change this to that," "change this to that," but say that "this" doesn't mean what you think it does, or you need to buy yourself a comma shaker and apply to your sentences, etc. In which case it is a big issue, because it's systemic. Nits are not systemic.)
Then you want to hit the middle-sized issues. The opening scene got off too slowly. The third scene didn't fit in with the story. The hero spends too long thinking and not enough acting.
Then, and only then, you can list the nits. (And if "this" and "that" are both grammatical, put down your reason for the change. "I would not have written it like that" is not a reason.)
I once got a five page crit of which eighty-percent or more was nits with bits of substantive criticism. I don't know. Wading through all the nits was not worth my time.