That's interesting. So in addition to making use of more RAM, by increasing the total memory space (RAM+swap) from ~10 GB to 18 GB, running Bitcoin Core is actually consuming more total memory. It's almost as if Core was (still is?) performing its own memory throttling (making its own decisions about which data to store in memory vs read/writing directly to disk) based on your total memory space. Maybe it does this in an attempt to optimize performance while processing the massive utxo set (if so, it would seem it's just not doing a very good job).