I have to say I am really disappointed about the state and performance of ExtJS 4. When the first test releases came out I immediately started testing it. The stabilization proces was long and painful though. During Sencha Con 2011, the grid performance problems were already known but they were presented like solvable in the near future. ExtJS 4 was still being presented as the most stable and fast ExtJS to date.
This thread and my own experiences however tell a different tale. Seeing how long it takes Sencha to solve the performance problems, I think the problems (most serious on IE but also other browsers) are here to stay and cannot be solved.
Because of the problems I have had to start porting back a very complex workflow management system I created for an international bank since they cannot switch to another browser than IE8. The performance of the system under IE8 is horrible both in memory usage as CPU usage, especially with components utilizing one or more grids. Choosing ExtJS 4.1 for the new system has cost my company a lot of time and money, and has caused the project to be seriously delayed. All that does not instil confidence in my client, for whom I have gone to great lengths to convince them to deploy the application using ExtJS4 (the previous version I created using ExtJS 3.4 which runs snappy).
I think it is time Sencha gives this problem serious effort since the credibility and businesses of developers using ExtJS is at stake. I know Sencha is working on it, but is also starting up a new product line every other day it seems. Everybody knows you cannot focus on too many things at the same time.