Every transfer runs at the speed of its slowest component: the source drive, the connection, the destination drive, or the file system's bookkeeping.
File count matters more than total size. Copying one ten-gigabyte file is fast; copying a hundred thousand small files of the same total size can take many times longer, because each one carries its own overhead.
Write caching explains the pattern where a transfer starts fast and then collapses. The first portion lands in a fast buffer; once that buffer fills, the speed drops to what the drive can truly sustain.
Network transfers add their own ceiling, and a shared connection or an old wireless standard will cap throughput regardless of how fast both drives are.