We did run UnixBench on one of our production servers before and after the patch. The tool indicated a performance decrease of a little over 20%.
We also routinely do timings on certain data batches that are run daily. We did
not experience significant changes in run time there; other factors (like concurrent load, I/O performance, networking traffic etc.) drowned the Meltdown patch impact in lots of statistical noise.
Bottom line, yes the patch makes your CPU slower in certain ways. But unless you can
measure that impact
in your actual application, you shouldn't worry about it too much. Not to the point where you would compromise system security, anyway.
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Corollary: Something I came to appreciate during the Amiga days, when there always was the next best and greatest accelerator board to be had (and I bought most of them), and people actually
cared for the access time of hard drives etc. -- anything below a factor of 2 doesn't really matter that much in your everyday work.