[McKayDavis]

> * Remove all unnecessary headers. Template expansion is slow, and preprocessing is even slower. Some of the standard includes (like `<regex>` and `<iostream>`) are notorious for slowing individual translation units to a crawl. `#pragma once` for your own headers also helps with cpp-time performance.

> * Forward-declare as much as you can. Forward type declarations mean that the compiler doesn't need to process all of `Foo` when it sees `Foo&` or `Foo`.

I've found the include-what-you-use (IWYU) tool [1][2] can help immensely with automating this process, especially on large code-bases.

It uses LLVM/Clang to analyze a .cpp file / translation unit and produces the minimal subset of exactly which includes are necessary and what types can be forward declared.

[1] https://include-what-you-use.org/

[2] https://github.com/include-what-you-use/include-what-you-use