[3.6.10 regression] Pasting in shell is SIGNIFICANTLY SLOW

This bug was first discovered in MSYS2, at:
1. https://github.com/msys2/msys2-runtime/issues/358
2. https://github.com/msys2/MSYS2-packages/issues/6501

However, it also reproduces on Cygwin bash, so I posted it here.

How to reproduce (copied from above issue):
Copy any multiline text and paste it to terminal. (singleline text pasting is
also slow, but not that significant)
For 3.6.9, the time cost is nearly ignorable, especially for bash.
For 3.6.10, both bash and zsh takes significant time (more than 20s for less
than 200 characters, 6 lines).

Note: the regression is slightly smaller in Cygwin than in MSYS2. The paste
speed in bash is about 1000 chars/s on MSYS2 3.6.9, 100 chars/s on Cygwin
3.6.10, and 10 chars/s on MSYS2 3.6.10. Cygwin 3.6.9 is not tested because I'm
unfamiliar with Cygwin, and failed to downgrade.

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