I just tried this simple tab test documented in this thread with 4.2.1.744-beta. While it's quicker than 4.1.0, it is still very noticeably slower than 3.4.1.1 in IE10, IE9 and IE8.

As others have noted, tab switching is essentially instantaneous with v3, but takes a noticable time (~.5 secs) with v4. Note that these tabs have only 5 labels and 5 textfields so this is *not* a stressing case at all.

V4.2.1.744-beta does *not* match the tab render and switching performance of V3.4.1.1