The short answer: about as fast as an internal SSD disk.
But you probably want some numbers. I used the Blackmagic Disk Speed tool, ran some tests and averaged the results:
|Internal SSD||600 MB/s||70 MB/s|
|400 MB/s||320 MB/s|
|400 MB/s||200 MB/s|
For the external disks, I used both a Samsung T1 500GB and a Samsung T3 500GB, both through a StarTech USB 3.0 hub, on my MacBook Pro Retina 13″.
Yes, you read that correctly. While the external SSD disks are a bit slower in reading, they actually seem to be 3 to 4 times faster in writing to disk than the internal SSD disk!