Depends on how it’s implemented. If they have a version of Proton that translates all x86 windows syscalls to ARM Linux, some operations could be extremely efficient.

There’s definitely got to be more overhead overall, though. Especially for devices with memory page sizes other than 4K, like the M-series Apple chips do (they use 16K as their page size), likely a VM will need to be sandwiched in there to ensure memory alignment. It’ll more fully be emulation and not just translation.