May 23, 2020
There is a slight performance hit when using the Dark appearance on macOS Catalina. It feels like there is just a tiny bit of input lag across the whole OS as compared to light mode. At first I thought it was my imagination, so I was happy to hear that Guilherme Rambo and John Sundell experienced the same thing on Stacktrace Podcast 084 at around the 16:50 mark.
Better yet, Gui found a solution! In System Preferences > General, setting the Accent color (not the Highlight color) to Graphite, the last option, disables the offending video compositing or blurring or whatever else is causing the delay. Unfortunately you're stuck with boring grey UI accents, but it's worth it for bit of extra responsiveness.
Graphite accent color
If you are a developer in the Apple ecosystem and haven't checked out Stacktrace yet I highly recommend it. This isn't the first macOS mystery they've unraveled for me.