Multithreading for Game Engines
IMO the Burst compiler that can "jobify" and multi-thread your code relatively painlessly is the most impressive piece of tech Unity has produced (disclosure: used to work there, though this was made after my time there). It's not that hard to fit into its constraints, and it often provides 2x-200x speedups. It's still mind-boggling to me that it's possible to get (a constrained subset of) C# to beat hand-optimized C++.