Opponent's taunt-time is TOO LONG

Playing Devil's Advocate here but the delay in offering the ability to respawn is part of the "team penalty" for being vanquished; you are deliberately pulled out of the battle for a short few seconds and your team loses your character and your contribution.

Generally speaking when you include the vanquish animation, the kill cam, the button prompt, respawn animation and then any travel time for you to get back to where the "action" is happening your team will generally need to do without you for maybe 15 seconds in most games-sometimes a little less. That's just enough time for the opponents to get a little advantage in objective based battles and just long enough for you to know that you're not able to help your team.

If they allowed you to nearly instantly respawn, say being able to get back into the action in 5 seconds or less, then there is essentially no penalty for being taken out by an opponent and there would be no incentive to try different tactics or to abandon overly aggressive (and possibly suicidal) game play style. For objective based game modes we'd probably see far more deadlocking at the first objective as both teams would be able to constantly cycle respawning players back into defending or capturing the objective.

In order for the "team penalty" for being vanquished to still be a factor allowing you to respawn faster would probably need to be balanced by setting the respawn point back a bit farther to increase the delay before you could rejoin the battle.