You might be confusing Gigabits for Gigabytes. 6 Gbps means 6 gigabit/s which translates to about 550 megabyte/s sequential in practice.
M.2 PCIe 3.0 x4 is 32 Gbps (4 GB/s), the same with PCIe 4.0 is about 64 Gbps, and PCIe 5.0 gives you 128 Gbps. 5/10/20 times faster than SATA 3, respectively.
Hard drives manage a bit above 250 MB/s nowadays, sequential. MLC and TLC SATA SSDs can usually saturate the interface, but QLC SSDs are generally much slower with write speeds between 50 and 150 MB/s.
M.2 PCIe 3.0 x4 is 32 Gbps (4 GB/s), the same with PCIe 4.0 is about 64 Gbps, and PCIe 5.0 gives you 128 Gbps. 5/10/20 times faster than SATA 3, respectively.
Hard drives manage a bit above 250 MB/s nowadays, sequential. MLC and TLC SATA SSDs can usually saturate the interface, but QLC SSDs are generally much slower with write speeds between 50 and 150 MB/s.