First thing is first, an NVMe drive is an SSD, the difference between an NVMe SSD and a traditional SSD is the interface used to access the drive's storage. A classic SSD uses a SATA III interface same as your typical spinning hard drive. This is because SATA III is cheap to produce, has a low system resource impact, and has been the standard for storage interfaces for years. However the flash memory in an SSD can reach speeds much faster than the 600MB/s theoretical limit of the SATA III connection. On top of this, flash memory can typically run in a bi-directional mode, (read and write simultaneously) where as SATA is strictly a one way street. This is where NVMe comes in.
NVMe based SSDs use a PCIe interface (yes the same kind of connection used for high end graphics cards and the like) to access the drive's flash memory. This makes NVMe drives more expensive to produce, however it also means the drives can run parallel read and write operations as PCIe is bi-directional and run at astronomical speeds. Current generation NVMe drives use a PCIe 3.0 connection, typically in a x2 or x4 mode. A PCIe 3.0x2 connection can run just under 2GB/s, and x4 just under 4GB/s respectively. This puts NVMe drives at speeds upwards of 2000MB/s compared to your typical SATA III SSD running under 600MB/s. With the upcoming PCIe 4.0 standard NVMe SSDs are only going to get faster, with projected figures at nearly 5000MB/s.
So if you are just looking for a number, your current generation NVMe averages around 1800MB/s faster than a SATA III SSD when running in x4 mode. The next generation of NVMe is expected to more than double that gap by being over 4000MB/s faster.