To be fair, I only learned about this from participating in the level solving contest round 1 and how we handled rankings for that round. Doesn't help that the post-level screen doesn't explicitly say "time taken to fulfill save requirement" (or similar, but I do understand how difficult it might be to phrase it tersely on such a screen). Granted, I don't play NeoLemmix much so I'm sure it had been talked about way back when the feature was first implemented to keep records of your "best" attempts, and thus is probably more familiar for the regular players.
You could make an argument that it might be of interest sometimes to also see other related time metrics, like time when last lemming exited or even total time consumed before leaving the level. Maybe not on the main post-level screen, but some way for the player to optionally go to somewhere that can show these other results.