As you can see this thread is quite old.
That said, most info in here is still valid.
USB will be the limiting factor in the read/write speed. It is really easy to set the root file system to a usb flash drive / usb hard drive.
I've used a USB HDD (regular spinning one) for a while, it has been more stable and faster then a class 10 SD card.
I tend to agree that an SSD is a bit of a waste. A regular USB HDD will probably do fine.
There's quite a few guides on how to set this up, continue searching on here