I ended up fixing this with the most unexpected solution from Stack exchange but it worked for me (unrelated to Xcode and does not apply to all models). I have an 2017 Intel MacBook Pro, and any Xcode would always have extremely delayed typing, no matter the size of the project that I opened. Then I realized that kernal_task CPU usage was very high. Apparently charging the MBP through the left Thunderbolt port can cause it to overheat and then throttle the CPU. External monitors also make this worse. Using the right-side port to charge fixed it. My CPU usage quickly decreased, and eventually I could type using Xcode again. Unfortunately it seems like many of you have M1 machines (which should not have this thermal throttling problem), so this would not be a solution for that. More info here: https://apple.stackexchange.com/questions/363337/how-to-find-cause-of-high-kernel-task-cpu-usage