No, it wouldn't. Thunderbolt versions 3 and 4 are giving you 4 lanes of PCIe 3.0 on an external connector. You can easily get 3GB/s from an NVMe SSD in a Thunderbolt enclosure, which is the same performance as putting a PCIe 3.0 SSD into your computer.
We don't yet know how fast the SSDs are in the new Mac mini, but the M1 Mac mini was using ~3GB/s SSDs, which is the same speed you get from PCIe 3.0, and realistically.. that's plenty fast for this discussion regardless.
We don't yet know how fast the SSDs are in the new Mac mini, but the M1 Mac mini was using ~3GB/s SSDs, which is the same speed you get from PCIe 3.0, and realistically.. that's plenty fast for this discussion regardless.