Aomei Partition Assistant (free):
http://www.majorgeeks.com/files/details/aomei_partition_assistant.html
Can be used to create a bootable USB drive from an .iso file of any Windows 10 version (including: Pro), bypassing the limited list of versions & USB devices officially supported by MS's "Win to Go". The only limitation: MS has crippled the ability to upgrade to a new major O.S. build number "in place".
So: MS hotfix updates can be applied okay, but Win 10 Insider builds can't be upgraded to a new Insider build "in place". Clean install required for that.
Obviously, even a USB 3.0 thumb drive would be slower than an internal SATA hard drive.
So: look for one of the faster USB 3.0 thumb drives, at least 32 GB in size.
That same Aomei software should also be able to clone the bootable USB thumb drive onto an internal HD/SSD and make it also bootable. Although, have not personally attempted such a cloning operation.
The fastest external portable & bootable Windows drive would be: an SSD connected via an adapter cable to a USB 3.0/3.1 port.