• SLC -> Single-Level Cell, i.e. 1 bit per cell
• MLC -> Multi-Level Cell, i.e. 2 bits per cell
• TLC -> Triple-Level Cell, i.e. 3 bits per cell
• QLC -> Quad-Level Cell, i.e. 4 bits per cell

The more bits per cell you store, the more dense and therefore cheaper your flash chips can be for a give capacity. The downside is that it is slower and less reliable since you have to be able to write and read exponentially more voltage states per cell, e.g. 2 states for SLC, 4 states for MLC, 8 states for TLC, etc.

USB 2 is 480 Mb/s, not 480 MB/s. 480 Mb/s is 60 MB/s, so the 500 MB/s from PCIe 2.0 x1 is quite a bit faster and is about the limit of what a SATA 3 interface could do. Also, sequential throughput isn’t nearly as important as most people think. Random IO, which NVMe drives excel at, will make a far more noticeable impact on real world performance.