UASP on an external USB3 enclosure with an SSD is on par with thunderbolt in my experience.
From what I have seen from Black Magic tests on YouTube, there doesn't appear to be a significant difference in Read/Write speeds between external SSDs using USB3 vs. thunderbolt. See here, for example, where he shows 300-400 MBps read/write speeds for both the thunderbolt and USB3 connections on the same SSD drive (go to time stamp 1:52):
[doublepost=1460746099][/doublepost]Just an update: I think I'll be sticking with the 1 TB SSD iMac, even though I'm convinced I probably would have been just about as satisfied with the 3TB Fusion Drive iMac. Some more speed and experience details below.
I ran the Black Magic Disk speed test (available for free on Mac App Store). The read speeds from an external USB3 hard drive (Seagate 3 TB Expansion hard drive, USB3) were ~90 MB/s (megabytes per second), while the read speeds from the internal PCIe SSD were ~1400 MB/s. Roughly 14x faster! (Note that, while USB3 is theoretically capable of speeds up to 625 MB/s, the bottleneck is actually the access speed of the spinning-platter hard disk, which apparently averages around 90-100 MB/s. See here for further corroboration of this point:
. Hence, if you are going to use an external hard disk, not much reason to spring for thunderbolt, as USB3 is still faster than what the spinning platter can deliver.
And check here to see someone else doing the same test I did on the internal SSD in a 5K iMac, also getting ~1500 MB/s read speeds from the internal SSD. Also note they got around 300-400 MB/s read speeds from an EXTERNAL SSD using USB3. So an external SSD with USB3 is about 3-4x faster than an external hard disk with USB3, but still a good 3-4x slower than the internal PCIe SSD in the iMac.
Finally, an internal 3 TB Fusion Drive apparently yields read speeds of ~580 MB/s.
, so, about 50% faster than an external USB3 SSD, and around 6x faster than an external USB3 hard disk, but still less than half the speed of the internal PCIe SSD.
My conclusions from all this is that, frankly, had I got the 3 TB Fusion drive, with its speeds about half that of the internal SSD but still much faster than the USB3 external hard disk, I probably would have been perfectly happy with it, and would have had 3x more storage space inside the mac, and saved ~ $500. On the other hand, I do feel good that I have "maxed out" the PCIe slot in the iMac, since, if I ever do choose to open it up in the future, accessing the PCIe slot is MUCH more effort than just accessing the SATA bay, and I could conceivably open it up (after warranty expires) and add another SSD in the SATA slot with little effort. Doing this today through OWC would cost about $600 for a 2 TB SSD drive. Hopefully SSD prices will come down further in another couple years. I also like that the iMac is silent, cooler (temperature wise) and that the PCIe SSD is (so I'm told) more robust and less likely to fail than a spinning platter Fusion drive.
With that Photos library, the system files, documents, and iTunes music library, I only have 210 GB remaining on the internal 1 TB SSD. However, I will probably offload the iTunes library onto the external (audio files aren't big enough for me to notice the difference in access rate over USB3 vs. internal SSD. . . ) and get another 100GB of headroom on the internal.
Also, having now run iMovie with the library sitting on an external 3TB USB3 hard disk, I’ve decided there is no need to run out and get a faster I/O external for this purpose alone. For some reason, iMovie runs buttery smooth even with the library on the external USB3 Hard disk. This surprised me, because Photos definitely did NOT run buttery smooth from a library on the same external disk: it took seconds to tens-of-seconds for photo thumbnails and videos to appear and then play when the library was on the external USB3 hard drive, and is super fast/smooth when its on the internal SSD. But I guess iMovie is somehow set up more efficiently. . . maybe it pre-loads more stuff into RAM. . . I don’t know, but the result is I experienced no slow editing. I’ve yet to export/transcode something sitting on the external. That’s the next thing to test. But others on Macrumors video editing threads claim that video editing is more CPU/GPU dependent than I/O dependent, surprisingly (to me at least).
The next external I buy will probably be either that “Fast” 4 TB RAID 0 hard disk drive (for $180), or an external SSD ($600). Probably the former. it could either be used to back up the full 1 TB and 3TB drives, or I could take advantage of its speed to store the iTunes and iMovie libraries, and use the slow 3TB external drive to back up everything until it gets too big. (I don’t think backup needs to be fast.)
