Some additional data points. Within same IE instance (different tab). Here are the reload times:

http://localhost/ext-4.0.0/examples/themes/index.html
5.3 s

http://localhost/ext-3.3.3/examples/themes/index.html
1.8 s


Also, changing the 'theme' via the dropdown is dramatically different. Under 3.3.3 it's almost instantaneous but under 4.0.0 it takes as long as a complete reload.