pip-tools: Slow compile

Describe the issue briefly here.

Environment Versions

1. OS Type: Ubuntu 19.04
2. Python version: Python 3.7.3
3. pip version: pip 19.0.3
4. pip-tools version: pip-compile, version 3.7.0

Steps to replicate

I have a new environment with the following requirements.in:

Cython
torch
torchvision

when I run pip-compile it just hangs. Here is the output of pip-compile -v:

Using indexes:
  https://pypi.org/simple

                          ROUND 1                           
Current constraints:
  Cython
  torch
  torchvision

Finding the best candidates:
  found candidate cython==0.29.10 (constraint was <any>)
  found candidate torch==1.1.0 (constraint was <any>)
  found candidate torchvision==0.3.0 (constraint was <any>)

Finding secondary dependencies:
  torchvision==0.3.0        requires numpy, pillow>=4.1.1, six, torch>=1.1.0
  torch==1.1.0 not in cache, need to check index

Expected result

Compiles normally

Actual result

Hangs on checking index?