I think I covered the reference level for cassette, although I'm sure someone would have eventually held up 8 Track as a viable medium.
Right- slow tape speed is the case against trying to achieve great fidelity in a small package- the shell can't hold enough tape to use tape of sufficient thickness to allow higher speed.
Back when cassette recorders were still very popular, people would ask why the specs showed -20dB- I told them that it's similar to a conveyor with shallow trays that are being filled with sand as they passed a spout. The flow rate of the sand is the recording level, the depth of the trays is the capacity of the tape to handle the signal and the speed of the conveyor equates to the tape speed- if the conveyor is too slow, the sand will overflow and if the trays aren't completely filled, there's still a bit of headroom.
Cassettes were never intended to be a high fidelity medium, they were always meant to be convenient.
Do you remember these?-
vintage Sony 3-head ELCASET recorder - the best
www.thevintageknob.org