Serious performance problems with Qt5 and X11

Question

We ported our application from Qt3 to Qt5. It runs smoothly under Windows but not under Linux (X11). With Qt3 there is no problem with Windows or Linux.

Inside the application there is a canvas of about 1000x800 pixels. A simple vector graphic is drawn onto the canvas. The user clicks into the canvas, holding the mouse button pressed an moves the mouse. Each mouse move results in a repaint.

We registered the milliseconds in each stage:

Start of MouseMove-event handling: 10581

call of update or repaint (makes no difference which one)

Handling of resulting Paint-Event: 10583

Painting finishes: 10584

return from update/repaint: 10687 (!)

I do not find any reason for this lag of 100ms (at each mouse move event!)

I need help!

Answer 1

In Qt4.8 the native graphics backend was deprecated. Remote X11 is no longer drawn with X11 calls but by painting onto a canvas and transmitting the result (a bitmap) to the client. This may result in larger bandwidth requirements and a slower when running X11 over network.

See also this