The entire decline of software quality that I felt in January wasn’t all due to a single buggy network-lookup service — but, unbeknownst to me at the time, a lot of it was. And that huge swath of problems that annoyed me every day disappeared instantly and completely as soon as I updated every computer on the network to 10.10.4.
It sounds like discoveryd was nowhere near ready for prime time last year for OS X Yosemite, but Apple shipped it for some reason. They've completely reverted to the older nDNSResponder, and they're sticking with it in this year's major release, El Capitan as well.
I don't know nearly enough about network tech to tell you why nDNSResponder is better than discoveryd, but it is a good reminder that sometimes it's better to stick with what's old and boring, and works, over something that's new and shiny.