Why is getBoundingClientRect slower in IE8?

I am using IE9 placed in IE8 standards mode with F12 tools to check my page. The script page uses getBoundingClientRectto determine what is visible in the viewport.

Using the profiling tool, I see that the call getBoundingClientRecttakes a lot of time. For example, 244 function calls took an exceptional time of 4.361 ms!

Is there a reason this feature is so slow in IE8? When working in IE9, Chrome, FireFox, etc. Function seems fast.