<8mph is where inexperienced / less able bicyclists begin to wobble unpredictably.
8mph is about the point where it doesn't feel too scary and fast to be passed as a pedestrian (NB. I have good vision, hearing and balance - your pedestrianism may vary).
12mph seems to be the design speed for curves/sight-lines on better British cycleways.
15.5mph is a reasonable speed to expect a pedal cycle or similar machine to cruise at on the flat.
20mph is the point where, as a cyclist, the speed differential with normal 30-40mph motor traffic starts to feel non-scary.
>25mph is where I'll confidently take a primary position in normal traffic flow.
On this basis, it's unreasonable to expect peds to mix with cyclists (or runners) without oodles of space, and for most cyclists to mix with motor vehicles travelling at more than 20mph. You can't change this by attempting to slow cyclists down. (Not to mention that attempts to slow cyclists down usually result in them rule-breaking, being unable to access your infra, switching to another mode, or riding somewhere else.)