Yes, very significantly, especially on old CPUs that don’t have hardware mitigations.

I've read the word "mitigation" so much (not only here, and not only you; so this comment is not directed at you personally) that I only hear doublespeak now: when people say mitigation I immediately think of a slab of concrete in the back of my car to compensate for not having any brakes installed. Please recognize for what it is: a design failure that is pampered upon. A mitigation is not a solution, it lessens the impact of the failure mode that was identified. Afaik, if the flushes and changes actually work, it is not a mitigation but a workaround: the failure mode cannot occur anymore. Mitigation sounds better than workaround though (and is probably the reason it was choosen).

Mitigation and work around mean the same thing.

Not really: mitigation lessens the effect, workaround avoids the effect.