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salmonchild-
I imagine it would be similar to an internal HDD. USB 3 would be a bit of a bottleneck but external HDDs are slower than that so I would think an SSD would be better. If you're using Thunderbolt or USB C I imagine it would be pretty good.
I did look at this when I thought I'd go down this route, and still may progress to it, but there are very few speed comparisons of external SSD and HDD speeds. I had a spare USB HDD so went with that but now I've got it working I'll probably look into an SSD at some point.
aggi
I mentioned on here about installing Windows 10 on an external drive but was having issues. I've worked out how to do it if anyone is interested (and mainly so I can refer back to this when I want to do it again).
Download the Windows Media Creation Tool from Microsoft. Run it and choose fresh install then opt to download an ISO image. Mount/unzip the image.
Use this http://markthetech.com/how-to-convert-install-esd-to-install-wim/ to convert the install.esd file to install.wim. The file is in the sources folder of the Win10 ISO.
Plug in the external HDD and run steps 1-3 from here http://www.partition-tool.com/resource/windows-10-partition-manager/install-windows-10-on-external-hard-drive.html (or use gparted or Windows Disk Manager or whatever you prefer, I found this quickest). (In theory you should be able to follow that guide through to the end but my version didn't seem to work for some reason.)
Download Gimagex from here https://www.autoitscript.com/site/autoit-tools/gimagex/
Run it and select Apply tab. For source choose the Install.wim file you created. For destination choose your external HDD. Click Apply. It will then spend some time copying the files across.
Open a command prompt as administrator
Run
where x is the drive letter for your external drive
You should now have a bootable Windows 10 drive. I got an OEM Windows 10 key for a tenner from here which seems to work fine. https://www.scdkey.uk/microsoft-windows-10-pro-oem-cd-key-global_1227-20.html?&site=uk
Performance is unsurprisingly slower in some instances (I'm using a mechanical USB 3.0 I bought a few years ago). Booting up takes about 90 seconds (as opposed to 10 on an SSD), the system can slow down a bit with a lot of disk access (installing updates, running programs, downloading, etc simultaneously) but generally it works fine and you don't notice it