thunderbolt 2/3 is very fast, but sometimes the hdd or ssd inside the thunderbolt enclosure isn't. a fast 7200rpm hdd would be ok for medium to low bitrate media, while an ssd would better handle higher bitrates and multiple layers of video. usb sata ssd's like the samsung t3 are pretty fast and usually cost less than thunderbolt ssd's.
I will have a fast 7200 inside of an external TB3 enclosure. What I am trying to figure out here is whether or not I should spend additional $400 to upgrade an internal SSD from 512GB to 1TB. And this would only make sense in case that storing the project files on the internal drive is a good idea.
Ronin is right Thunderbolt 3 is very fast but if you put any hard disk drive on any TB interface you have a lousy transfer rate because it is only a hard disk drive.
There are external enclosures with TB and an SSD inside but then you are generally paying a lot more than the Samsung T3 prices. What surprised me is the apparently the Samsung/USB is faster than the LaCie device because the LaCie is only TB 1. If anyone know of an external enclosure for or with a single SATA III SSD and Thunderbolt 2 or 3 interface let me know. Here is my Samsung T3 and look at those sequential read/write rates which are what you really want for video editing.
Almost as good as an internal SATA III SSD
I now own 3 Samsung T3 for editing on my laptop. I have all the project files and media on one drive and can easily walk the project from my laptop to my desktop and back again.
That $400 would buy you a 1TB Samsung T3
i think the ssd apple offers in the new 27" imac is super fast, much faster than sata ssd found in normal external ssd's. so it should be fine to edit from the internal ssd if you want.