Actual transfer speed & fluctuation depends, amongst other things, on size & number of files being transferred.
If you transfer just one very big file, the speed will stay pretty much constant, but when transferring a whole bunch of small files of varying sizes, the speed will be all over the place.
Degree of fragmentation of the storage device is also a relevant factor, as mentioned by previous poster.
Phillip - I connect a USB 3.0 HD to one 3.0 port, a USB 3.0 thumb drive to the 2nd 3.0 port, and select ONE file to copy HD-to-thumb - and the speed immediately drops to under 10MB/sec. If I'm copying 3.0 to 3.0 device - why is it exactly as slow as plugging in a 2.0 thumb drive to receive the data? I have seen data xfer at 80MB/sec - once or twice in other copies - and in this case the HD is a 1TB backup onto which I had initially copied all the data from a 500MB drive and have only copied a score of new files to it since that initial backup. (The current problem - from HD to essentially empty thumb - would seem to eliminate the fragmentation issue too). Stymied!!!! When I first encountered this problem, I searched online for hours and finally found a reference to a setting which - by default - turns off 3.0 speed (I'm paraphrasing badly) for other efficiency gains, with details as to how to locate that on the computer and turn 3.0 on. I did that, briefly in a few instances saw 80MB/sec speeds... and now once again it's a dog - and I cannot relocate the switch or the info (it comes up in a nice requester stating what the on/off setting provides and allows me to activate 3.0). Thanks for reading & helping!