Through a series of coincidences, I discovered that this performance cliff is somehow related to the application being active or inactive (if I keep the app in focus or move the mouse around over the window, performance remains high). My performance question is essentially solved but the question about what "kernel.release.t6020" actually is and an explanation as to why it occurs would be helpful for other users.

After a bit more searching and investigating, the root cause of my performance problem was caused by App Nap. I only realized this by happenstance while looking through my Instruments capture where my main thread was running "mach_msg2_trap" and the callstack involved "rbs_acquire_appnap_assertion". Realizing this, I went to use https://developer.apple.com/documentation/foundation/nsprocessinfo/1415995-beginactivitywithoptions with https://developer.apple.com/documentation/foundation/nsactivityoptions/nsactivityuserinitiatedallowingidlesystemsleep and now everything works perfectly. It would be great if Instruments would more obviously surface App Nap as a state change in the process so it's easier to understand what's going on.