I agree timing is hard on IR. I personally skipped one long, complicated question on my actual exam. However, I wouldn't skip the Multi-Source Reasoning because there are usually 3 questions tied to one Multi-Source Reasoning - so it's worth a quarter of the IR.
Also keep in mind that within each problem, there are multiple questions - and you must get them all correct to get full credit. Get one wrong, and you get the entire question wrong.
I would advise that you just keep practicing - do a lot of IR problems. Once you have more experience with IR questions, you will figure out when you're taking too long on a problem for the actual exam.
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