I have been using for several years bootable Thunderbolt 3 SSD's (on Linux).
As those are equivalent with 4 PCIe 3.0 lanes, exactly like the internal M.2 type M SSD slots, there is no performance difference.
Only the latest laptops with Tiger Lake, and the laptops with Ryzen 5xxx to be introduced soon, have PCIe 4.0 M.2 slots, so they will be able to have faster internal SSD's.
I expect that the interconnection standards for external devices will also continue to be improved, restoring parity with the internal devices.
As those are equivalent with 4 PCIe 3.0 lanes, exactly like the internal M.2 type M SSD slots, there is no performance difference.
Only the latest laptops with Tiger Lake, and the laptops with Ryzen 5xxx to be introduced soon, have PCIe 4.0 M.2 slots, so they will be able to have faster internal SSD's.
I expect that the interconnection standards for external devices will also continue to be improved, restoring parity with the internal devices.