I have never come close to 100Mbps scanning speed and I haven't got a large amount of or complex data. I recall it being around 30Mbps scan speed, but I have seen it slower than that.
So what type of hard disk have you got (SATA1, SATA2, SSD), how bigh a Cache memory, what format type (FAT32 or NTFS) how much data is on the drive and what is the make up of that data, mainly media files, very small files, etc. ?
Here is one that would have an impact, how fragmented is the drive and or have you recently defragged it ?
That could impact on the Persistent cache and slow your scan until you build up the persistent cache over a few scans.