So I have noticed this recurring issue with my cards. Currently I have 2 32GB Sandisk Extreme Pro CF cards and one 16GB Sandisk Extreme card. The Extreme Pros are 90MB/s advertised, and the Extreme is 45MB/s advertised for read speeds. Here is my problem:
Normally when I come back from taking pictures, I just tell Lightroom to directly import from the card and handle the file copying to my hard drive, etc. However recently I tried to manually copy the DCIM folder to my computer and noticed that while the initial speeds were fast (60+MB/s), after a while it would drop to 11-12 MB/s! It was a easy reproduced several times, in which for one of the 32GB cards once it had transferred 4 GB it would drop to the slower speed, and then for my other 32GB it would be 10GB before dropping. The 16GB Extreme did not exhibit this problem. Now here's the weird part: After it copied however many GB before switching, it would pause for a second or two before switching. And if I cancelled the copy, and then tried to copy another folder or restart the operation, it would stay "stuck" in that 11-12MB/s speed. The only way to fix it would be to unplug/replug back in the reader or the card.
I am currently using a Sandisk ImageMate USB 3.0 All in One reader, and this reader with my cards was producing this issue. I had an older USB 2.0 version of the same reader, and it did not produce this issue (it stayed at about 22-25MB/s the whole time).
I have tried this in Windows 7 using TeraCopy as well as in Windows 8 using the default file handler.
To me it sounds like the reader's messed up, if the USB 2.0 reader is reading at a consistent (comparatively slow to the USB 3.0) 22-25MB/s for all 32GB. However the fact that the "limit" before it changes speed is different on both of the 32GB Extreme Pros and does not happen on the 16GB Extreme make me think its the cards.
Does anyone else have any input or suggestions as to what is causing this?