IE7 has built-in anti-phishing features, and for every site it
loads, it checks. This slows down loading time.
Next, interface. Notice that IE7 looks nicer than IE6. Another
thing is ClearType technology. ClearType makes it slower than
anything else.
The last thing is of course user problem. If you open lots of
programs and expect IE7 to load fast, you can dream on. Also, if
you have less than 512MB of RAM, don't complain.
You can turn off anti-phishing feature in IE7 to speed up the
loading of IE7. Not too sure if you can turn off ClearType.
Other things to do include close unused programs, add more RAM and
check for malware problems.