Now, I kind of suspected that the answer would be yes, but I didn’t actually know any details – and as a general rule of thumb, I don’t like to just assume that what somebody else told me is true (or even that I am actually remembering correctly – I’m not getting any younger, you know) so I figured we would run a little experiment and see what happened.
I then ran this test in each of the 4 rendering modes in IE9, and for good measure I also threw in a test of the latest Firefox and Chrome. These were the results of running the test in an uncontrolled environment, a single time, on my computer – I am not attempting to make a scientific statement about absolute performance, just to gain an understanding of whether there are differences which are large enough to matter.
Wow. The outcome seemed really clear to me: when in IE9 or IE8 standards mode, IE9 performance is very clearly competitive with other modern browsers. However, once you drop to the two oldest compatibility modes, performance drops by an order of magnitude – that’s a big deal!
Since Compatibility View is the default for the Local Intranet zone, I would therefore strongly consider a program of enforcing the inclusion of X-UA-Compatible headers to opt in to one of the modern document modes in order to maximize the performance of your enterprise web applications, which is precisely the advice I gave to my customer.