Issue with echo & source to /dev/stdin

[Anand N <anand.mail@gmail.com>]

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Hi All,

My zsh has become very slow after upgrading to Mac Big Sur OS.
It was nvm that was causing the issue. So I researched and found that nvm
can be lazy loaded with the `--no-use` option.

However, even after using --no-use, it was still slow. The reason is a
simple echo command to /dev/stdin. It is taking >5 seconds. Below are the
lines that are causing the issue

```
[ "_$( . /dev/stdin yes 2> /dev/null <<'EOF'
[ $# -gt 0 ] && nvm_echo $1
EOF
)" = "_yes" ]
```

We noticed that even a very basic echo to /dev/stdin is taking more than 5
seconds. For example

```

time ((echo abc) | (. /dev/stdin))

/dev/stdin:1: command not found: abc
( ( echo abc; ) | ( . /dev/stdin; ); )  0.00s user 0.00s system 0% cpu
5.087 total

```

This happens only with shell built-in echo and doesn't happen with
/bin/echo.
We tried many options and a more detailed discussion can be found in the
below github issue
https://github.com/nvm-sh/nvm/issues/2387

Looking forward to your help. You can directly reply in the github thread.
It will help others as well.

Thanks,
Anand

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