It’s not. Actual tranquilizer guns are used when dealing with animals, not people. You dart the animal and wait up to half an hour for it to kick in. When we’re talking about a bear wandering around a subdivision and knocking over dumpsters, that’s fine. You keep people out of the way, and give the animal time to keel over, then you take steps to maintain its safety while you transport it elsewhere.
Tranqing people isn’t that different. If you dart someone, you can be looking at up to thirty minutes before they’re actually under. That’s thirty minutes for them to realize they have a dart in their neck, freak out, and alert the entire base, and start a manhunt, or start shooting hostages, whatever applies.
Also, tranquilizers aren’t safe, not really. Overdosing will kill the victim. Dosage is calculated based on a lot of factors, including weight, age, metabolism. When you’re dealing with wildlife, you can make sure the animal gets proper medical attention if you overestimate, but that’s not an option in combat.
You can’t really speed up the rate that the tranquilizer works, safely. Because, if you miscalculated the dose, then it will go lethal that much faster. With a slower onset, if something does go wrong, you’ve got time to get medical help to the victim. Except, none of this works, if the entire point is to remove someone quietly while infiltrating a facility.