I use this infrequently with digitizing negatives, and there are a bunch of issues that can come up, and often this is a bit of "close your eyes, change a few things, cross your fingers and hope". Some specifics I've found: (1) format the card in the camera before you start (I realize the card is not to be used, but when you plug in a camera to the computer, the first thing the (some? all?) driver does is try to itemize all the photos, and it can appear to hang for minutes if there are a lot of photos), (2) reboot before testing (sadly even in 2016), start with camera off, plug in, turn on, (3) check and remove any competing drivers or other software that might have had drivers for it, (4) check in Device Manager and look for the warning symbol on any device on the USB bus, and try to resolve it as needed. The issue you have with progressively slower transfer though, does not sound familiar. In most cases for me it either works, or fails, not something in the middle. If practical try on a different PC as well, and if you know someone try a different camera on the SP3. Finally and not pretty, you might clear out your lightroom preferences and reset to basically a fresh install (be sure you know how to put them back); Adobe recommends that, though I have yet to see it truly solve a problem, it will be high on their list if you ask them.