I have no such issue on winXP Pro SP2 and my system is no speed freak.
What other security software is on your system ?
With 4.8.1201 there is a log created by the rootkit scan C:\Program Files\Alwil Software\Avast4\DATA\log\aswAr.log.
Open this using notepad, at the top 'header' information there is a start time, at the bottom, 'footer' information is the end time, using these two how long does the scan take ?
Example mine started at 14:49 and ended at 14:50 so that is one minute and even that is likely to be less as there is no seconds recorded in the start and end time to get a more accurate picture.