As a chemical engineering student, I have realized through the years how important a timely answer to a question might be. In my personal experience, pending concerns over one's own understanding of a non-intuitive defined concept in phyisics or chemistry (for example, when you're giving your first steps in dealing with thermodynamics-related issues) will put a limit on your attempts to achieve further advances in the subject.

Of course, there are always also those free-minded folks who can "go on" with a course, even though they didn't accomplish to grasp previously-discussed ideas in their entirety. But as a matter of common sense, those "holes" in their knowledge database about that field will sooner or later become apparent. Without loss of generality, this pedagogical rule can be extrapolated to any other rigorously enough field, as this is a general feature of the scientific and technical learning process.

The thing is, they will become apparent usually sooner than later, with embarrasing or serious consequences. And it's one's responsibility to be sure that's not the case.