I have this awful issue when running dual monitors with this set up. I get the worst kind of performance/lag, and im not sure what the problem is. When I disable either of the monitors in the display settings, I get exceptionally fast performance so it's either something with the GPU drivers, or the computer itself not being able to handle it?
Set both monitors to the same resolution and refresh rate. plus its Ubuntu, so I'd say driver support is not like Windows. I'm no Ubuntu guru so I can't really help you, but this quick google looks promising