Yes, it can exhibit non-optimal performance, and in some specific cases (regex's especially), extremely poor performance, but that's not the same as being poorly designed and implemented, especially given the breadth of the thing.
The ABI Nightmare - The C++ committee has this extraordinarily weird and strict rule: never break the Application Binary Interface (ABI). If a better algorithm or memory layout is discovered, the standard library cannot adopt it because doing so would change object layouts and break existing binaries. The worst part is that this ABI is never defined, so you always HEAVILY pay for what you DON'T use.
std::regex - the Programming Language Joke of the millennium. Even an interpreted language regex engine runs faster.
std::map, std::unordered_map - outdated, badly-designed and slow crap that is beaten even by high-school coders writing map data-structures.
No bounds checking. And Undefined Behavior by Default for operators like std::vector::operator[]
std::iostream - bloated, expensive design, std::vector<bool> - another joke.
Silent Iterator Invalidations causing unpredictable memory corruption.
No deprecation strategy. There are FOUR callable wrappers. At-least, have the courage to say @DEPRECATED.
No Standard Networking.
Missing System Utilities - nothing for process management, standard cryptography, or basic command-line argument parsing, etc.
To be honest, this is just the common complaints - if you run through all the stdlib features, there are dozens of severe problems. Which all the smart people know about, but are forbidden to fix - because of ABI!