Thunderbolt 3 is 40 Gb/s of PCIe + DP — basically exposes your bus out through a Type C connector. USB theoretically tops out, as of last year, at half tha, though in practice you won’t see anything faster than USB 3.0 (if you’re lucky) which tops out at 625 MB/s, before all the USB protocol overhead.

TL;DR: USB drives are really really slow.

> though in practice you won’t see anything faster than USB 3.0 (if you’re lucky) which tops out at 625 MB/s

625 MB/s is 5Gb/s; the switch from bits to bytes seems to be a way to exaggerate the difference from Thunderbolt speeds quoted in GB/s; at any rate, USB 3.1 Gen 2 (equivalent to USB 3.2 Gen 2×1) 10 GB/s cables and device support isn't hard to find; I don't know that anyone had USB 3.2 hardware (controllers, or cables supporting the new 2×2, 20Gb/s, mode) yet.

625 MB/s is "really really slow"? I don't know any HDD which can deliver more than 100 MB/s. And for SSDs there's also USB 3.1 Gen 2.