I have done some performace test on the GXT 3 vs GXT 2 TreeGrid.
Since the buffergrid that the TreeGrid in GXT 2 extended don't exists anymore in GXT 3 you would expect that performace to be bad if you have a large dataset. And yes it is. In my testcase it was about 7X slower. It depends on the number of rows in the TreeGrid.

I'm verry surpriced that Sencha allowed this kind of performace degradation in one of the core widgets in the library. I would expect more effort in this area, speacially to not repeat the total performance faliur of Ext JS4.0.

If anyone want my testcase, please let me know.
TreeGrid
One text column
1 Root
5000 Children

FF: Create TreeGrid + Attach to DOM + ViewReady
GXT 2: 650ms GXT 3: 4500ms