DNS lookup painfully slow on only one network?
Thanks for all the suggestions. I verified that this is indeed only a problem on this machine, so I figured it must be something misconfigured. 

I found that I had Cloudflare's 1.1.1.1 (and associated backup and IPV6 addresses) as manually-set DNS servers for only this network. When I removed these addresses, my lookups became blazingly fast. 

The mystery is now: why is 1.1.1.1 so slow? I set these as my DNS servers on my iPhone and it's nice and fast. I'll mark this question as solved because my original question has been resolved, but it have generated another bigger question.