I left `bitcoind` running for some hours in a very powerful machine and can confirm that now it is going very fast again, so the bottleneck is something in the Raspberry Pi.

In the powerful machine I am running it with the default settings, i.e. no bitcoin.conf, except for `-datadir` which I point to the USB storage SSD. In this powerful machine, `pmap` reports a total memory usage of `bitcoind` of about 14e9. It seems that 8 GB of RAM is at least in part responsible for the bottleneck, no matter what you put in `dbcache`.