It is not a bug that there are overlooked optimizations in some platform features. Windows has a ton of slow features. Starting a process, for example, takes forever. It is the responsibility of application authors to write their performance-sensitive critical path in such a way as to avoid bogus platform behaviors. This goes for Linux, which has more than its fair share of brain damage, as well as Windows.
I generally agree with you. Having worked on lots of cross platform software, a big part of that job is to work around quirks of the underlying platforms, which can be significant. However in this case, it's not that Firefox was introducing the usage of these APIs and was then starting to have performance problems. They used the APIs without problems when suddenly Defender came along and slowed them down by orders of magnitude when they had been working fine for years.
