I've already mentioned that I'm running approx in the laptop and I configured the FreeRunner to access the laptop's cache. Here are the other customisations needed to have a decently working apt:
# cat /etc/apt/apt.conf.d/99freerunner APT::Install-Recommends "false"; Acquire::PDiffs "false";
The rationale is that recommends would bloat a system that is supposed to be small, and pdiff requires more CPU, memory and disk space/time than it actually saves in bandwidth.
Thanks to Michael Banck and Peter Palfrader for helping me to find out how to disable pdiffs.