To swim well is to move efficiently and harmoniously through the water. Fast swimming naturally follows from efficient swimming - thus fulfilling more traditional criteria of “good swimming” (as measured by the clock).
The opposite of efficient swimming is sometimes called “fighting” the water. Beginning swimmers often fight the water (almost by definition), but advanced swimmers aren’t immune. I often catch myself doing this when I’m fatigued and trying to hold a pace slightly beyond my comfort zone. I’ve paid much more attention to not fighting the water since I started doing marathon swims. You might be able to get away with fighting the water in a 50, or even a 200, but in a marathon this is death. A relaxed, efficient stroke is essential.
On days when I’m not feeling so hot, I try to forget about going fast and just focus on relaxing and swimming efficiently. If I’m working out with a team, this may require slight adjustments to sets.
For example, say the coach assigns a descend set - 4×200 descended 1-4. Instead of trying to go faster on each 200, I’ll try to hold the same pace on each one, but with progressively less effort. The only way to hold pace constant while using less effort is to become more efficient. Incidentally, I think these types of sets are useful as a warm-up to a long swim - or during the few days leading up to it.