I'm not familiar enough with the internals to say for sure, but I do know Python has a penchant for dicts. Perhaps it looks up the function each time, whereas Go can be made aware of the function at compile time? That wouldn't be without cost.
Part and parcel with its love for dicts, Python data structures tend to not be shaped like C typically expects. Whereas Go data structures are, at most, lightweight wrappers around C-style structures, and are often even directly equivalent, making passing data about as simple as passing a pointer. Python may be getting bogged down in some kind of marshalling operation. That isn't cost-free either.
Part and parcel with its love for dicts, Python data structures tend to not be shaped like C typically expects. Whereas Go data structures are, at most, lightweight wrappers around C-style structures, and are often even directly equivalent, making passing data about as simple as passing a pointer. Python may be getting bogged down in some kind of marshalling operation. That isn't cost-free either.