It's well-known for competition programming in C++ to use the following simple I/O optimizations:

• ios_base::sync_with_stdio(false) which shouldn't ever be a problem as you shouldn't use C I/O in c++ ever
• cin.tie(0) (except for maybe interactive problems)

(they should be prefixed with std:: but competition programmers use namespace std which is bad style but probably ok for cp)

Here's an ancient Dr Dobbs article on it https://www.drdobbs.com/the-standard-librarian-iostreams-and-std/184401305
also https://codeforces.com/blog/entry/6251

Should this be its own article? So we have a convenient reference instead of tracking down CF blogs.