Update to this: It appears that there is a serious amount of packet loss on the RPi4 when eth0 receives data, but not when it sends data. Confirmed with iperf3 between the RPi4 and my Fedora desktop. It can reliably do full gigabit when sending from the RPi4 to my Fedora desktop, but it can barely even do 20 Mbps in the other direction, and it does a lot of retries (about 30 retries per second) while doing so. I've tried disabling RX offloading with ethtool -K eth0 rx off, but that didn't help.
As such, this is very unlikely to be DNS-related, and more to do with the RPi4's eth0 being unreliable for receiving data. Perhaps it's the real reason why I was having this issue earlier: Raspberry Pi eth0 instability due to EEE being enabled by default - #5 by daigennki