Also imagine VAR taking 5 minutes on each sin bin decision on top of what we have already

If they had a sin bin and somehow decided the way to implement it was "we will review every sin bin decision and then put the player in the bin" rather than "we will put players in the bin and then review the decision, inviting them to rejoin the game before their time in the bin is up if the binning was unjust" that would be insane.