I'm not affected by this at the moment, but I had a look into this.
I'm not using the jquery adapter, rather than ext-base.js (add my changed sources later).
The loadtime of both versions feels the same.
The Ext.3.4 Version seems to have a layout problem or whatever (no floating toolbar, panel with forms visible only after first click).
But then I started clicking though the tabs. The feeling, between my finger click down the mouse button and the tab is going active is long. Checked against the 3.4 version and I didn't had this impression.
Now I needed to have a deeper look and used chrome speedtracer. The event running times are higher, which could confirm the longer duration to activate a tab.
This are not really arguments to confirm that this is a bug. Its more a additional description for sencha devs.
Yes, the slowness is not as apparent in newer browser versions (although there is still a noticeable delay in them that wasn't there in 3.4).
However if you run the 4.0.2 example in IE8 the tab panel becomes effectively unusable for any consumer product (ie. consumers of products written using ExtJS 4) where IE8 is a large percentage of the install base.
Yes tab switching is slow, but also render time is very slow in certain browsers.
In 3.4 when you click a tab the tab gets the visual highlight immediately and the corresponding panel is immediately shown. In 4.0.2 there is a delay updating the visual indication of what tab is active as well as showing the corresponding panel.
In IE8 there is also a very big difference in page render time. From the time you refresh the page to the time the page is render and able to be interacted with is about 8 seconds in my tests with IE8. In 3.4 this load time is only about 2 seconds. These tests were done from a local web server so there is no network delay affecting the results.
I'm seeing a slow down as well. I have added blakel's tests to my web server. Chrome and Firefox are mostly passable although it feels a bit sluggish. IE (6,7,8) is fine in the 3.4.0 version but it lags in the 4.0.2a version.
A cursory glance at the code shows that Ext 4 is using buttons for the tabs and I think Ext 3 used list items. It looks like every time the tab is switched in Ext 4 all of the tab buttons are going through a layout cycle. Then again I'm no expert so it could be something else.