Template:Build Documentation apt-cache
Using an apt cache will greatly improve build speed when building several times in a row (debugging, development).
If you are interested, click on Expand on the right.
In short: just get an apt cache running and set the http_proxy environment variable.
sudo apt-get install apt-cacher-ng
Be sure to have a firewall, so not the whole internet can use your apt-cacher-ng service.
sudo http_proxy=http://127.0.0.1:3142 ./whonix_build ...
If you are building inside a non-Whonix VM, you could use an apt cache on the host. In that case adjust the IP accordingly. (And manually test it is reachable.) If you are building inside a (Whonix) VM, you can just install the apt cache inside the VM and the point to a localhost apt cache.