Slow download speed when using port due to "failed to resolve: <host>:<port>"

[de-code]

Say I have a local website on port 1234, then calling httrack more or less like:

httrack \
    http://localhost:1234 \
    '-*' \
    '+*localhost:1234/*' \
    --mirror \
    --verbose \

...then the download is very slow. Enabling --debug-log shows frequent "failed to resolve: localhost:1234".

Running the site on port 80 and removing the port when calling httrack solves the issue in that case.