It's clearly a trade-off. I have found that as I gain experience, reading code becomes faster, even for domains that I'm not familiar with, and it's nearly always worthwhile to understand the problem.

Moving fast and breaking things adds to the technical debt. The word debt implies that you will have to repay it (or at least, someone will have to repay it). So moving fast now means slowing down later. Sometimes this is desirable, sometimes not.