Buying a USB C cable wouldn't help, Lightning is limited to USB 2.0 speeds anyway.
for data transfer speeds, I can tell you that there's really 2 main units. b/s (bits per second) and B/s (bytes per second). There are 8 bits in a byte, 8 kilobits in a kilobyte and so on. So, if your ISP advertises a download speed of 8 Mb/s (eight megabits per second), your browser or download client would, in ideal conditions, show 1 MB/s (one megabyte per second). You can use speedtest.net to get your connection speed in Mb/s, but real-world performance varies by program, bandwidth usage, latency and many other factors.