Created attachment 1112624 [details]
Thread dump of Eclipse while attempting to run debugger.

Description of problem:
Eclipse 4.5 (Mars) runs unbearably slowly under GTK3.  The problem manifests most significantly for me in the debugger, although I do notice it to some extant when importing projects.

Version-Release number of selected component (if applicable): 
% rpm -q eclipse-platform gtk3
eclipse-platform-4.5.1-6.fc23.x86_64
gtk3-3.18.6-1.fc23.x86_64

How reproducible: Unsure.  It seems to be working fine on a laptop I have, but on my desktop, performance is consistently poor.

Steps to Reproduce:
1. Install eclipse-platform and eclipse-jdt
2. Import a project, create a breakpoint on a class, and then run something that would hit that breakpoint.  E.g. a unit test.

Actual results:  The debugger never even connects to the JVM or if it does, it's so sluggish as to be unusable.

Expected results: Working debugger.

Additional info:  If I use the same workspace, same ~/.eclipse, and just run `eclipse eclipse --launcher.GTK_version 2` everything works and is quite fast.  I've taken a thread dump (attached) and when using the VisualVM profiler, I see that 33% of the time is spent in org.eclipse.swt.internal.gtk.OS._gdk_window_get_origin[native] with an additional 28% in org.eclipse.swt.internal.gtk.OS._gdk_cairo_region_create_from_surface[native]

Please let me know any additional information you might need and I will be happy to provide it.