An external USB3 SSD will run faster than any internal spinning drive. Getting BootCamp installed on a USB external can be a major hassle though.
I run BootCamp on an SSD in a
Delock Thunderbolt enclosure which I previously used to boot my Late 2013 iMac. It's a great little enclosure and reasonably priced.
Because Thunderbolt-attached drives are recognized as SATA devices it makes installation of BootCamp a lot easier as well.