What's python's LOAD_FAST bytecode instruction fast at?

Question

https://docs.python.org/3/library/dis.html#opcode-LOAD_FAST

I have been wondering, why is it specifically named fast and not just "load"?

What alternatives could be used instead that are "slower"?

When are these alternatives used?

Answer 1

All the _FAST opcodes take a variable index to access a local variable. This is fast because it's a simple index into an array rather than a name lookup, which might have to loop through multiple contexts.