As far as I can tell, Priority #1 is backwards compatibility. For example, std::regex can be slower than launching PHP and running the regex there¹, but that won't be fixed because it would require an ABI break. I'm not sure how that happened, as neither the poor performance nor the unwillingness to change ABI were unknown. Performance seems to not have been the top priority in the design of std::unordered_map either, which requires reference stability and therefore boxing. This got added in 2011, 18 years after the birth of the stl.

¹: https://cor3ntin.github.io/posts/abi/