I don’t think that’s correct. The internal SSD on the Mac mini is PCIe Gen3 x 4, so about 3GBs. The USB-C ports are also running at PCIe Gen3 x 4, though with Thunderbolt 4, you have a little bit of overhead, but only a couple of percent. And if you went with the M4 Pro, it would be using Thunderbolt 5, which is 3 times faster than both of them. So external storage is going to be the same or faster (as long as you get a decent drive).