That varies wildly from path to path. My evening ride after work is a 24.5 mile round trip on a local MUP which I average 19+ mph solo on (with a timer that doesn't auto-pause for the 8 or so intersections each way). I'm hitting speeds in the high 20s, and I'll go for several miles without seeing another human (deer, turkeys, turtles, pheasants, aggressive groundhogs, OTOH). Try doing that on a sidewalk. I, like you, love riding on the shoulder of a highway because of the long uninterrupted high speeds, but this path ride basically gives me the same thing in a location close to the office.

Now if you take the Charles River Greenway through Waltham and Watertown, I wouldn't ride that again on a bet. I feel rude riding anything over 13 mph on that path.

If you're going point-to-point in a major metro area, you also have to consider what the alternative to the path is. If the roads around the path have a bunch of buses and stoplights and stop signs at busy intersections, frankly the road riding might seem more like sidewalk riding to me than riding on a MUP where at least I can maintain a steady pace.

My sense is that the people who automatically turn their noses up at all paths miss out on a lot of really good riding.