[naasking]

I don't think performance was a motive for effect systems in Haskell, it was more about making effects easier to compose, understand and debug.

[ookdatnog]

I don't think it was a primary motivation, but at least the slowness of mtl is mentioned as one of the motivations for the existence of the library.

https://hackage.haskell.org/package/effectful#what-about-mtl