I'm tempted to agree that Spawn Interval may be a better choice than Release Rate. But I'm not sure how I feel about the current max, 99, becoming 4, as it would feel psychologically better if the lowest Spawn Interval was 1, though that might be a bit dense of a rate for lemming release.
The main advantage of Spawn Interval, IMO, is that it's a completely different way of measuring the spawn rate of lemmings, instead of the New Formats redefinitions of what, for example, a release rate of 50 means - part of the reason why I opposed the un-duplication of the redundant release rates. Once Spawn Interval is explained, it is also slightly easier to understand, since Spawn Interval directly correlates to the thing it affects in-game.
The main disadvantage is that this would change big numbers from being fast to big numbers being slow, which might take some time to adjust to in gameplay situations.