Although Cycle Craft says 18mph max for a shared path I think that's far too fast when mixing with pedestrians, dogs, small children on bikes etc.
If I'm on them I keep myself to 10 mph or less, unless I'm the only user in sight. That's partly for my own safety, as those paths are full of people not looking or on the phone, cyclists who don't know that they should pass on the left, 1km long dog leads, and they have surfaces similar to the Bermuda Triangle in a hurricane.
I have only rarely had the "You should be on the path" whinge from motorists. I usually just ignore them or give them a pained, fed up type look. I'm not responsible for their pig-ignorance.