I have been running 19.10 (64-bit) with MATE desktop on a RPi4 as my main computer for about 3 weeks now. I run the system from an SSD.
In general, the performance is similar to the previous system I was running (raspbian lite with MATE desktop), but some things are considerably faster.
The most outstanding is Thunderbird, my email client. With raspbian it took well over a minute to start up; with ubuntu, it's just a few seconds. Another improvement is with some large spreadsheet files I use. There is about a 30% to 40% reduction in the time it takes to open them with librecalc.
One small annoyance. I set the screen to blank after an hour of non-use. This works, but the backlight stays on. I know this was a problem with raspbian on the RPi4, but this was fixed in an update a month or two back. So hopefully this will also be fixed on ubuntu soon.