It's a funny feeling opening hacker news and seeing your project at the top :)

In iTerm2's case it's because of bad decisions about a decade ago. The data model is read & written on the main thread. The main thread is solely responsible for drawing operations if you use the standard drawRect: interface on NSView. That means that a slow draw blocks it from processing more data. Other work, like reading from the socket and parsing bytes into tokens was offloaded onto other threads years ago. Unwinding all the assumptions about single-threadedness is nearly impossible. The only way to reduce the main thread's workload is to use a GPU renderer because building the data structures the GPU needs can be done off the main thread after a quick copy of the necessary state.