ansi-term in EmacsForMac 26.1.2 on Macos slow when not in foreground

[Matt Mello]

I am running mpv, streaming music from the web.

When I run it in ansi-term, and when the frame is not in the foreground,
the music breaks off every few seconds, as though the performance of
emacs/mpv has been compromised.
Of course, as soon as I pull it to the front, it is fine, and so long as I
have it in front, it seems fine.

I can do the same thing in the Terminal or even inside of IntelliJ IDEA's
Terminal plugin.  When I run it in those, and when their window is in the
background, it still works smoothly.

I'm running EmacsForMac:
GNU Emacs 26.1 (build 1, x86_64-apple-darwin14.5.0, NS appkit-1348.17
Version 10.10.5 (Build 14F2511)) of 2018-05-30
on macOS High Sierra 10.13.5.


So, since it works smoothly in Terminal and IDEA, it doesn't seem like the
issue is related to mpv.  But, it does seem related to emacs+ansi-term.

Any idea what I can do to make emacs+ansi-term not have this performance
issue when not in front/focus?

Thanks!
-- Matt