Does top show plasmashell running at 100%?
No, plasmashell is
running at 1 .. 4% cpu even while I'm navigating through
the menu.
Also, check to make sure you don't have overlapping panels on any
of the
displays.
Ok, I have just one full-width panel at the lower edge of my external
monitor.
Moving the panel to another edge of the external monitor does not eliminate
the delay problem.
However, when I move the panel to the LVDS manually (by using Panel
Settings and
click-selecting/moving 'Screen Edge'), the delay problem
- is gone on the menu (panel on LVDS)
- is still present for the widgets on the primary display (widgets on
external monitor)
B.t.w., the popup menus and widgets react with delays of 1 ... 3 seconds.
I should also mention that the Compositor is set to use the XRender
backend.