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The only sensible way to use a USB boot drive is to get a USB 3 enclosure with UASP and put whatever SSD you can be bothered buying in there.
UASP is really important if you're going to do this. The enclosures cost about a tenner on amazon.
Bear in mind that TRIM won't work over USB but that isn't really an issue. Just keep 30% of the drive free so it can handle its own garbage collection.
Ok folks... long story short... I can't instal W10 on my iMac anymore... crazy idea/performance hit running it off a USB drive (flash)?