Consider reading through FA or whatever topic you're doing first, before doing those Qbanks. That way, you can annotate only what's important and what's missing from FA.
I'm not too sure about Kaplan because I don't use it, but with the other Qbanks, it's often times not necessary to make sure you understand 100% of every single word in those explanations. It would take entirely too much time if you were trying to annotate every single word of new information into FA.
Personally, what I do is -- for the ones I get correct, I'll just read through the answer choices and the "take home point" (or whatever the Kaplan equivalent of that is), and make sure that is indeed my rationale, and maybe jot down a sentence or two of any "cool" information I find.
For the ones I get incorrect, I'll read through the explanation in its entirety and summarize/reword the concepts and write those down -- no more than 3-4 sentences, and that's if I really had no idea what the concept was (which doesn't happen often).
I get ~70-80% on UW right now, and it takes me ~1 to 1.5 hours to go through each question (so maybe about 2 to 2.5 hours at most per block)