As in anything reliant on something else, the maximum speeds will always be at the lowest speed connection. If your engine has 400hp, but your gearbox is stuck in 1st gear, you won't be able to drive very fast even with all that power.
there is no maximum speed of a virus scan. although some are faster than others, but thats down to the way it has been written and what its scanning for.
they will read as fast as your system will let them, you notice i say system as it relies on the speed of the cpu, the amount of cpu its allowed to use, the ram and the HDD.