What type of USB does the Form 1+ use? USB 2.0 should be good enough. The average USB 2.0 data transfers are about 30-35MB/s, so at that speed you should be able to transfer 2GB in about just a little over a minute. So in 10 minutes 20GB, and so on.
I don’t remember the size of the SD card in the Form 1+, but even if it was 16GB (I think it’s either 8GB or 4GB), it should take less than 10 minutes to fill it.
So there are 2 possibilities, either the SD card is very slow or the USB connection is not USB 2.0 capable.