Depends on the fight. Length and adds being a big part of it. Single target fights it drops in output. Same with medium length fights where you miss out on that 3rd-4th use by 30s-1m.
yeah I mean I can’t say I know from experience but I have seen the variation lol. It’s not an insignificant amount.
Nor is it game-breaking to not have it either. Yes every little bit helps, but we had/have people downing mythic bosses and topping parses without it