This might give you some answers:
http://www.webpagetest.org/result/13...15Q/1/details/
Speed Issues
1) No GZIP compression
2) No caching of images or static javascript
3) First Byte Time is slow (nothing you can do other than change hosts/server)
To help with speed issues read this article:
https://vborg.vbsupport.ru/showthread.php?t=267588
To integrate vB headers into your custom pages read this article:
https://vborg.vbsupport.ru/showthread.php?t=228112
Not sure how you went about integrating the headers. There are a few different ways. I personally believe the one linked above will give you the best overall results.
Whatever way you did it, you have javascript and css styling being loaded down in the <body> when it should all reside in the <head> of the page. This will also slow page rendering in browsers. Issues like this will typically cause more issues in Internet Explorer than other browsers. Which is at least one of the reasons you may see a longer delay in pages rendering in IE.
I think if you can use the practices outlined in the 2 articles above and implement them on your site you will see some great improvements in speed and rendering.