The major changes in branch prediction mechanism seem to be present since Zen 3:
The fifth iteration of AMD’s Zen core architecture is set to launch later this month. Zen 5 marks nearly seven years of Ryzen processors, starting with the release of Zen in 2017. Since then, we’ve gotten Zen+, Zen 2, Zen 3, and Zen 4 at the heart of the Ryzen 2000, 3000, 5000, and 7000 …
hardwaretimes.com
AMD states that the fix will positively affect Zen 3/4/5 performance. That points to a fact that Windows was not optimized to utilize improved mechanism since Zen 3, while Linux was.
Then there may be another Zen 4/5 branch prediction issue on the top of that. Zen 5 makes the biggest changes to the decoders as well (the biggest since Zen 1).
And after that there might be VBS performance hit.
Yeah, all the reviews and other performance tests were inaccurate. There has always been some unused performance on the table due to the piece-of-sheet OS.