New file loop_tk.py solve the responsiveness problem, at least for this example, by using the asyncio loop as it is and doing tk updates in a callback loop. It works with both SelectorEventLoop and ProactorEventLoop.

I was inspired to try this, instead of my planned modification of _run_once, but Yury's post on uvloop http://magic.io/blog/uvloop-make-python-networking-great-again/.  I realized that any modification of asyncio.XyzLoop would only work for that class, where as a standard callback loop would work with any sufficiently compatible loop.

Yury, if you happen to read this, could you try loop_tk.py with the two uvloop lines added and get_event_loop uncommented?