On Feb 25, I had to optimize a client's website. A plugin I built for them was struggling with performance issues when liking entries posted by other users. The database query took an average of 1.2ms to perform. Nevertheless, the API response took up to 1.2s(!) in the worst case for 63 bytes of JSON data.
I started investigating and noticed that the official WordPress REST API seemed to be at fault. So I did the first thing I could think of, ripped out the voting script and placed it in a raw PHP script that loads the bare minimum of WordPress I needed to perform my action.