Bizarre Import from SD Card Issue

Just sent this in to tech support. We'll see what they say, but I'm curious if anyone else has experienced anything like this:

I've run into a bizarre bug when importing photos into a catalog collection from a SD card with a USB card reader. Namely, if importing to referenced files from the card to a location on my hard drive via C1 (not storing the files in the catalog itself) then the import gets extremely slow the SMALLER the image preview size that's set in C1's preferences. For instance, importing 41 Nikon D750 RAW files at the following preview sizes proceed as follows:

5120: 32 sec.
3840: 32 sec.
2880: 32 sec.
2560: 37 sec.
1920: 46 sec.
1680: 55 sec.
1440: 92 sec.
1024: 240 sec.
640: 337 sec.

Of course the larger previews take longer to generate, this is just for the actual file import. At the large sizes, like 5120 pixels, the import outruns the preview generation (as you'd expect), but as you get down into the slow zone around 1440 and below, both the importing and preview generation are very jerky. Watching the progress window it's almost as if they're fighting one another.

These times are all with OpenCL off. Turning it on increases the times by about 10% (I normally have OpenCL disabled anyway). If I download the files from the SD card to my desktop (which is plenty quick) then import them via C1 to the same referenced location on the same drive, then it works quickly no matter the preview size. If I import from the SD card directly into the catalog itself, then it also works quickly no matter the preview size. I've tried this with different catalogs, different SD cards from different cameras, and it's entirely reproduceable just by changing the preview size. What could be going on here?

2008 Mac Pro, 8-core 2.8GHz, 12GB RAM, 6TB internal software RAID, ATI Radeon HD 5770, OS X 10.10.5 Yosemite.