Comment by Pannoniae

1. Compilers barely do even basic optimisations such as interprocedural register allocation when faced with non-trivial code. You often also need the most aggressive optimisation settings, LTO or even PGO enabled for many of these.

2. Virtuals are, with the exception of PGO, mostly a black box i.e. you get a hard optimisation boundary, no inlining at all.

3. The C++ standard library is usually comically slow (yes, even compared to Java/C#/the likes) so if your project uses std::vector and the such instead of specialised libraries, you've already lost at the beginning.

4. If you don't pay attention to performance from the get-go, the approximate amount of autovectorisation you'll get is close to zero. Some compilers are better than others (Clang>MSVC for example) but I've seen codebases with 8 figures of LoC where the number of vectorised divides/multiplys was like less than ten when you dumped the object listing. In the whole program.

5. Since aliasing and other optimisation barriers (you didn't use restrict or manually hoist, did ya?), it's not uncommon for large C++ programs to spend a third of their runtime doing atomic increments because shared_ptr is supposedly cheap and who cares about lifetimes anyway.

6. If you're targeting Windows, the default new operator / malloc is also comically slow. Luckily that one is fairly easy to fix with installing mimalloc and deploying the hijack dll, but the negative effects on cache by the fragmented allocations is also significant.