The last few times I have ran a scan they have been getting a lot faster.
I have made no changes to the system inbetween the scans, and I am afraid it is not scanning properly.
Is this something to worry about or is there an explanation for it?
Scan time depends upon your RAM space and hard drive capacity. By faster, how much time do you mean it takes to complete?
Also let us know the version details of your McAfee programs;
Virus scan version :
DAT version :