Internet Explorer performance is dependent in part on the number of simultaneous connections. One connection / one element can be downloaded - this includes graphic elements. Each element on the page has to be downloaded. If a page is only html text, the default number of connections available to IE, 4 connections, is adequate. What if the page has buttons, lines and lots of other graphical elements. The number of elements that can be simultaneously download is limited to 4. You can experiment with the the number of connections for IE with the following registry hack. Milage may vary. Experiment to see how it works for you.

Hive: HKEY_CURRENT_USER
Key: Software\Microsoft\Windows\CurrentVersion\Internet Settings
Name: MaxConnectionsPerServer
Type: REG_DWORD
Value: 20
Name: MaxConnectionsPer1_0Server
Type: REG_DWORD
Value: 20

These values will not exist. If you are going to experiment, I would create the values and increase the value by 5 or 10 until you see no improvement. Pick complex pages to experiment with and don't forget that pages are cached.