I have noticed this problem while testing the new LV2 version of amsynth.
When dragging any of the knobs on amsynth's custom (Gtk2) UI, the UI is very unresponsive. Both the plug-in UI and the application's UI become unresponsive.
It appears that GTK's run loop is being delayed somehow, resulting in events being fired at a much lower rate than expected.
The same problem is not present with the DSSI version of amsynth (which uses the same UI code) nor is it with other hosts tested - jalv and Ardour 3 beta 5.
Interestingly, Calf Monosynth's LV2 Gtk2 UI does not exhibit the same problem, so I investigated further...
I found that changing amsynth's LV2 UI so that it only calls the LV2UI_Write_Function from a timer (instead of from the "motion-notify-event" handler) the problem goes away.
But this does not feel like the correct solution, as it will slow down parameter updates. The UI really should be able to call LV2UI_Write_Function whenever it makes most sense (i.e. while the user is dragging.)