Thanks for sharing.
My guess is that ~3% slowdown would be a negative side effect of the "fix" and probably no much chance to be avoided in the future for current CPUs. I've seen on some sites even ~4% slowdown on average, for example here:
https://www.phoronix.com/scan.php?page=article&item=zen-3-spectre&num=1Regarding random reboots, this should of course not happen and it would be good if checked on some other, non-Asus motherboards.
1) If with other motherboards with same AGESA these random reboots do not happen, then it is most likely due to a bug in Asus BIOS.
2) If it happen with others motherboards with same AGESA but not with different AGESA, then it is up to AMD to fix the bug, and Asus to implement it in new BIOS.
3) If it happens with everyone, then it is AMD and/or MS to blame for issuing a very bad fix, but this is a bit unlikely.