UI freezes for ~10 seconds on :! external commands on Windows

[craigmac]

Oni Version:
September 15 2019 Build 64 bit Windows exe

Neovim Version (Linux only):
Operating System:
Windows 7 64-bit

Issue:
Running :! to perform an external command freezes UI for about 10 seconds before displaying results (e.g., .exe files on Windows)

Expected behavior:
Much quicker, < 1 second max response time.

Actual behavior:
~10 second freeze of UI before showing results

Steps to reproduce:

1. Open Oni2.exe
2. Hit :!

[oni-bot]

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