The crux of the complaint is not game speed, but time flow speed (i.e. the fact that when guests/keepers/animals move on normal speed, it takes months for a keeper to check an enclosure, prep food and feed the animals. A slow-mo button wouldn't solve that. I haven't counted exactly, but in their lifespan the animals have like what 20 meals for some of the species.
Obviously you need to some kind of faster pace of time than 1 for 1, but I'd prefer to have animals live for a larger chunk of time of a zoo build. Now it feels like they are gone and dead in the blink of an eye. If you let them die of old age in your zoo that is. The game incentives you to trade them away before that because it's more efficient. Release to wild is a few button presses. Removing a dead animal requires a vet to remove it. And if the inspector spots it, you pay a fine (not that it matter when you have a million in the bank, but that's another problem), so releasing to wild is always the more efficient play. I find this a bit odd, that when you put in the trade center or release to wild, it's instant, but when you put them in their exhibit it requires a staff member to carry them. Longer life span (in game time by slowing time flow, not simply adding a few years without changing time slow) and a dead animal sanctuary/graveyard would be much nicer.