It's very unclear to me how much of a performance hit there is for most desktop use-cases.
My most recent CPU I use is a desktop i5-8500 which doesn't feel faster than my older i7-3930k aside from disk access [0], both of which are impacted by the vulnerabilities. I practically never use the i5.
Until recently my daily driver was a MacBook Pro that came out in 2013, and I don't really remember a before / after change in perceived performance or responsiveness – I haven't done any scientific benchmarks, but that computer never felt slow, and still doesn't.
There's also this Phoronix benchmark [1] that compares performance with and without `mitigations=off` on linux. There are some tests where there's a very big difference, but mostly, to me, it doesn't look that crazy.
In the Firefox benchmarks they do say that there's a big hit, but then I probably don't read the graphs correctly, nothing looks particularly worrisome to me.
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[0] The i5 has some NVME as opposed to an older, regular SATA drive in the i7. Still, I run a lightweight Linux install on both, so drive speed is never an issue for me, everything can fit in RAM.
[1] https://www.phoronix.com/scan.php?page=article&item=spectre-...