Hi, I've identified why drawing is lagging with GTK+3. https://bugs.launchpad.net/inkscape/+bug/1723247
It's because of GTK3's motion event compression: https://bugzilla.gnome.org/show_bug.cgi?id=702392
Adding a * gdk_window_set_event_compression (window, FALSE);* in SPCanvas::handle_realize() makes things much smoother.
At 1st I thought it was because the events were sitting in the queue for too long. So I added some timing code to measure the latency between when a motion event was generated in GDK to when SPCanvas::paint() is called. Actually, I detect bursts of mouse moves or redraws and only use the 1st for latency measurements since there might not be a 1 to 1 relation between motion events and redraws. I was seeing a 4 to 10ms latency for head (GTK3) but only 0.5 ms for 0.92 (GTK2).
I thought I was on to something, but this mislead me for a while. Finally, I saw that the # motion events and redraws were 10x higher for GTK2.
I haven't stayed up to date with the GitLab migration. I tried to push a patch to my branch simdgenius_inkscape, migrated from Bazaar, but access is denied. I just requested project access, so appreciate it if someone grants it.
-Yale