I'm connecting an Early 2011 MacBook Pro 13'' which only has USB 2.0 port via its Thunderbolt 1 port to a Kanex KTU20 Adapter which successfully allows me to connect USB 3.0 cables to this old Mac Book Pro. I tested transmitting 3 Gigabyte of files to and from an exernal HD which takes 24 (eHD>MBP) resp. 45 (MBP>eHD) seconds.

I therefore calculate bitrates of 987 resp. 526 Mbit/s, which is way below the possibilities of USB 3.0 - 5000 Mbit/s - or Thunderbolt 1 (10 Gbit/s), but is of course much faster than the bitrates I get with USB 2.0, 202 (117s) resp. 275 Mbit/s (117s), which itself is also way below the possibilities of USB 2.0 - 480 Mbit/s.

Now, I connect this Thunderbolt-USB 3.0 Adapter via USB 3.0 to a Sabrent DS-Rica USB 3.0 Dock for all sorts of peripheral devices, among them am external screen via DVI cable. Up until then everyting works fine: If I connect my eHD to that dock and test the speeds again, the transmission times are the same and all peripherals, like the mouse and keyboard, work as expected.

But when I connect that screen, the USB 3.0 bitrates plummet to approx. 307 Mbit/s (77s) in both directions, on the screen there is about half a second of lag and some graphical errors (most prominent: the Desktop Dock at the bottom of the screen is not transparent anymore and its animations are annoyingly slow, ~1s), in addition the mouse pointer behaves laggy and double clicks when I intend a single click, key strokes appear delayed on screen and internet pages (via Ethernet cable which goes into that Sabrent dock too) are loaded more slowly. Over all, everything is majorly laggy when I connect the screen. So here are my questions:

Some additional information first:

  • My MacBook Pro runs macOS 10.11.6 (El Capitan), the Sabrent Dock utilizes DisplayLink drivers, the hard drives inside my MacBook Pro are connected via SATA 6 and 3 Gbit.)

  • When I connect the DVI cable via a DVI-Thunderbold [DisplayPort] Adapter and everything else to the dock, there are no display lags, but the mouse still misbehaves... If I connect everything directly to the MBP, no problems occur.)*

  • According to the system information utility of the MBP there is a USB 3.0 port when the adapter is connected... So I assume that I can expect corresponding speeds...

Diagram of current setup:

MacBook Pro ------> Kanex Adapter ------> Sabrent Dock ------> Screen
               |                     |          |         |
         Thunderbolt 1            USB 3.0       |        DVI
                                         other Peripherals,
                                       (mostly via USB 2.0)

                                         as well as Internet via Ethernet cable and Audio via Jack


Main Questions:

  1. Why am I experiencing these lags? Which part of the setup is responible for them?

    My guess is, that the Sabrent Dock is overwhelmed with the information it has to process. Or: USB 3.0 cannot transmit all that data (i.e. the USB 3.0 bit rate is too low?!). The graphic data introduced by the DVI screen is to much for one ore both of them.

  2. Are the bitrates I experience normal or too low?

Secondary questions, that arise from this:

  1. If it is the Sabrent Dock, could this be a problem with the drivers? I read somewhere that "DisplayLink has a history of being incompatible with Mac"...

  2. Or is this just a cheap dock? Could this and 3. be resolved by getting a better USB 3 dock (which doesn't use those drivers)? Any suggestions and personal experiences are much appreciated!

  3. Or is it the Kanex Adapter itself? Is there in this case even a way to connect this old laptop via a dock to peripherals or do I have to connect everything directly to it? (Of course I could and maybe should just get a newer laptop...)

  4. Or am I missing something, an it is somethng completly different?

Please instruct me if I should provide more data with other setups/tests to rule things out...

  • @Ramhound Because I'm connecting USB 3.0 to the MBPs Thunderbolt 1 port via adaper. Thunderbolt 1 has a bitrate of up to 10 Gbit/s. Or does this still not allow for USB 3.0? According to the system information utility of the MBP there is a USB 3.0 port when the adapter is connected... So I assumed that I can expect corresponding speeds... – CJLM Sep 7 at 4:10

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