I had this issue with "sudo apt update" or at trying to install something with composer in the WSL. The issue has been connected with IPv6. So I have disabled IPv6 and the requests where working much faster.
In Ubuntu (not Windows 10) disable IPv6 that way: Add the following lines at the bottom of /etc/sysctl.conf:
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
Run sudo sysctl -p to reload the /etc/sysctl.conf settings.
To disable IPv6 only for apt-get:
Put the following contents in it:
Save the file, and you're good to go. If you want to force IPv6 instead, change the 4 to 6 in the file name and settings. To choose between one of them, test with the fast method and see which works better.
In Windows 10 I could fix my issue for the Bash by the execution of a small program from Microsoft: https://support.microsoft.com/en-us/kb/929852