They will use up to whatever bandwidth/speed is available but the smarter ones back off when they detect you are using the system so it does not effect performance as much.
Generally if you start a scan then leave it to do the scan, you can use it meantime but will only make it take longer.
As an example when I scan customer systems they range from an hour to 3 depending on disk size and how fast the system is.
The reason why you are @ 100% usage and 10MB/s is when reading a bunch of small files performance drops heavily as opposed to one large file.
One file that is 1GB is size will take exponentially less time to read and scan than 2000 files totaling 1GB.
The Windows directory consists of many small files and so do most program installations. The AV software is not limiting you but your disk is.