Environment

pip version: 20.1.1
Python version: 3.7
OS: Debian Linux

Description

python3 -m pip download --dest /tmp numpy==1.18.0 --no-binary :all: --no-use-pep517

finishes immediately, while:

python3 -m pip download --dest /tmp numpy==1.18.0 --no-binary :all:

takes a while to complete.

We originally noticed the slowdown in https://issues.apache.org/jira/browse/ARROW-8983.
The command python -m pip download --dest /tmp pyarrow==0.17.0 --no-binary :all: also gets stuck for a few minutes with "Installing build dependencies ... ", and increased CPU usage when pip downloads sources of numpy, a dependency of pyarrow. Interestingly, in case of pyarrow, adding --no-use-pep517 does not help.

Is there an inefficiency in pip download, or it's a known behavior and is considered WAI?

Thank you.