I've never thought about this, but how big are the performance penalties for Spectre mitigations vs not having Javascript JIT? (Just to be clear, I'm perfectly

I've never thought about this, but how big are the performance penalties for Spectre mitigations vs not having Javascript JIT? (Just to be clear, I'm perfectly happy with the performance of my old-ish machine, so I have no motivation to disable those mitigations.)