Witrhout any details whatsoever it's hard to judge why it's slow but several different factors can make a scan slow. If the drive has disk-read errors, in which case do a chkdsk /r - or it could be that there are lots of compressed files that need to be unpacked.
Make sure that under the Scheduled Scans section that there is a check mark next to 'Use minimal resources' as in the picture here (click to magnify)
If so try unchecking it then click Apply and the scan will run faster.
Not knowing your software version means the settings may not look exactly the same as above.
Locking this discussion as it is over (3) years old.