Copying from an SD card in the built-in slot or from a flash drive connected to USB would only copy at just over 3MB/s. Copy to/from computer via the USB port was 25-30MB/s. I requested help from support and was told that exFAT formatted drives are currently slow (being worked on), but FAT32 would improve speed.
As far as I can tell, Windows 7 only allows formatting as exFAT or NTFS, so I formatted as FAT32 in another machine. Copy times jumped up to 9.3MB/s; about 1GB in 2 minutes.