Why is there a speed increase when using a USB 2.0 device with a USB 3.0 port?

I know this question, or similar questions, have been asked before, but still I am curious…

My laptop has two USB 3.0 ports and—on occasion—I will plug in a Western Digital external hard drive which uses USB 2.0. I have also used this same drive plugged into a USB 2.0 port and—having tried both of these configurations—I can say that I notice a bit faster transfer (roughly 20 MB/s more) using the USB 3.0 port. This doesn’t make sense to me.

Is there any reason for this?

This doesn’t make sense to me.

Is there any reason for this?

I noticed this as well when I upgraded my Mac Mini to a model with USB 3.0 ports on it; my older USB 2.0 drives would copy data noticeably faster. The logic I believe is not all USB 2.0 controllers are the same.

The way I understand it, USB 2.0 speed is spec’ed to be a max speed; not a minimum and not a promised consistent speed. Meaning when USB 2.0 first came around, it was faster than USB 1.1 but nowhere near the speed of 480 Mbit/s promised. As USB 2.0 chipsets improved, speed and compatibility improved.

Simply put not all USB 2.0 controllers are the same. Heck, no kinds of controllers—SCSI, FireWire, PCI, etc…—of any and all types are the same. Or a better direct tech analogy is why can some gigabit ethernet controllers transfer speed better & faster than others? It’s simply improvements in the controller.

So looking at USB 3.0 specs, it simply seems that USB 3.0 controllers just perform better overall and the benefit is that the USB 2.0 standard it downgrades to is a better performing implementation of USB 2.0.

This is all anecdotal and conversational, but I did find this AnandTech article on “USB 2.0 flash drive performance in a USB 3.0 port” that supports this claim:

The real-world transfer times show that while USB 2.0 flash drives do, in fact, perform faster when 'up-plugged,' this improvement is very minor, to the point of being nearly or not at all perceptible. That said, if you have USB 3.0 ports available, you can shave a few seconds off your transfer times by sticking your USB 2.0 flash drives into them.

And this answer on another Super User question backs that up as well; mind you the accepted answer to this question says there is no difference but it’s clear to me there definitely is is:

Actually, yes, it will be faster by a small margin. You will only see gain if the device in question can dish out a higher bandwidth over another interface like ExpressCard or PCIe. for instance a modern 7200 hard drive in a external enclosure could more than saturate the USB 2.0 port. If the enclosure is a USB 2.0 device, it will be operate with more of its bandwidth when plugged into a USB 3.0 hub, but not nearly as much as if it was a USB 3.0 to USB 3.0 device to hub link (with a USB 3.0 cable).