Just loaded the themes example into IE9 across my LAN in about 4 seconds, virtually identical to FF 3.6.16. Takes about 2 seconds into Chrome on the same client. The client is a win 7 64-bit ASUS U30JC-B1 laptop.
Also, loads into IE8 via http://localhost/ext-4.0.0/examples/themes/index.html in about 5 secs. This is a win 7 64-bit Ultimate desktop with core i7 CPU (2.67 GHz) and 12 GB RAM. Of course, on this machine Chrome only takes about 1.5 sec.