When I click on Internet Explorer, it takes approximately 15 seconds for my
homepage to fully load.

However, if I then close IE and re-open it again, my homepage is displayed
in 3-4 seconds.

If then shut down IE and try again later (e.g. 30 minutes), the pattern
repeats itself.

Any idea why, and how to speed things up from the start? Nothing I've tried
seems to work.

NOTE: I'm running IE8, but had the same problem with IE7 -- and with Firefox.