it causes much more than a hash table lookup: it causes an attribute lookup, which is ...
1. an instance __dict__ lookup,
2. a __getattr__ call (itself involving another hash lookup)
3. a class property lookup
4. superclass property lookups (if there are any superclasses)
If it was just a hash table lookup, it would be LuaJIT2 fast. But it's more like 10 hash table lookups and indirections, and that's assuming you don't actually try to hook any of this machinery to do something.