There is a misunderstanding. "Slow device" in the POSIX sense is about unpredictability, not maximal possible bandwidth. Reading from a spinning disk might be comparably slow in the bandwidth sense, but it's actually quite deterministic how much data you can shovel to or from it.

A pipe on the other hand might easily stall for an hour. The kernel generally can't know how long it will have to wait for more data. That's why pipe reads (as well as writes) are interruptible.

The absolute bandwidth number of a harddisk doesn't matter --- in principle you can overload any system such that it fails to schedule and complete all processes in time. Putting aside possible system overload, the "slow device" terminology makes a lot of sense.

Seeking a tape also takes an unpredictable amount of time; and so is seeking a disk, for that matter (IIRC, historically it was actually quite difficult for UNIX systems to saturate disk's througput with random reads).