Why does 64-bit mixing cost 30% more CPU?

[metamorphosis]

This option was always available in earlier versions of Tracktion, but had negligible performance impact (not even sure if the difference was audible).

In Waveform 13, when turned on it consumes ~30% more CPU on the tracks I'm working on.
To the best of my knowledge this gets used when combining track outputs, so it's a pretty limited-use functionality even in a larger project. There's no 32-bit->64-bit audio processing when supplying to plugins, because they all take 32-bit inputs (even if they, like Izotope's ones, use 64-bit internally).

And it's not an expensive operation (32-bit to 64-bit conversion), even on 100 tracks. So why is it consuming so much CPU? Answers from the Devs only please.