I just tried to download gtkmm-dev and it was taking a bit longer than I thought necessary ... like 30min+ and not done.
Doing it manually (wget) worked fine.
It's not a fresh install (5.2), so I already have/had some of the required deps. I'm thinking there is a recursive dep somewhere, but I'm not sure.
I'll keep trying to figure out the exact cause. Maybe this is familiar to someone else.
EDIT: I'm trying from repo.tinycorelinux.net.