Disappointed I didn't see fairly reasonable explanations around processor security bugs which impact broad system performance such as Spectre and Meltdown. This has a significant impact on Linux and Microsoft has even outlined that these fixes impact their performance (there have been many more security bugs identified since): https://www.microsoft.com/security/blog/2018/01/09/understan...
What I find most disappointing is that even processors manufactured TODAY contain these flaws and have performance hits put into the firmware. This issue is so fundamental to how CPU caches work, that there really is not a true performance-neutral fix.
The first (private) disclosure of the Spectre vulnerabilities was sometime in mid-late 2017. Partial hardware mitigations launched in 2019 for some Spectre variants, where the mitigations could be easily patched in to already taped-out designs.
Beyond those easy wins, even if foundries weren’t slammed by COVID delays, I don’t think it’s realistic to expect that we’d be any further along? Most of these sidechannels require deep rethinks of the execution pipeline and speculation engines, and fundamentally redesigning those systems, taping out those designs, and finally manufacturing those chips was always going to be a process that was measured in the better part of a decade.