Even though Javascript is quite dynamic, Python is much worse. Basically everything involves a runtime look-up. It's pretty much the language you'd design if you were trying to make it as slow as possible.

Just like Smalltalk and Self.

Which can change on the fly anything that is currently executing in the image.

Also after breaking into the debugger, the world can be totally different after resuming execution at the trap location.

Then there are nice primitives like a becomes: b. where all occurrences of a get swapped with b.