Something I have never had or seen explained is why a file XFR (for example) from HDD to HDD will start off at 90 MB/second and taper off to 65 MB/second?

I do lots of large photo file XFR's and would like to optimize for a better flow if I can. Are there some services I can ditch I don't need or some other tweaks or settings to check?

The example I give above is of a 124GB of photo files and folders transfer I am doing as I type from a set of WD Black 640GB HDDs' in RAID 1 to a single WD 1TB Black. Both internal SATA drives.
It started at 90-somthing GB/second and settled down to 65-ish.

Thanks