That is kind of what I was thinking too, and cross-compilation is still the fastest way to build for a different target.
Using distcc networked compilation instead of cross-compiling is reasonably fast too and easier to set up if one isn't familiar with either.
the most frustrating part with having to compile a custom kernel is the maintenance burden (packaging/updating etc.), and not the time it takes to build…
I had a similar issue with networking modules for calico (k8s cni) on both rpis and jetson boards…
well, the tricky detail here (which we do not mention in the post, our bad) is that we got the raspbian config (cp /boot/config ... .config && make oldconfig) which includes most modules, and that's why it took more.
But yeap, good point about using the -j flag, it really accelerates the build!
One nit: this should only take about 40 minutes on a Pi 5, assuming you're compiling with -j6 to use all the cores.
(Still faster to cross-compile)