I did a quick test here, rendered an image in 4k with LightMix, denoising, and bloom and glare on vs off.
The time difference was 2 seconds (10 min render time limit, ~12min total time spent).
The rays/s difference was 2.8%.
I understand this has some impact on the overall render speed, especially when rendering animations, but at the same time I don't think it's significant. If anyone disagrees, or has a sample scene which renders much slower with b&g enabled, please feel free to share here.