Why is cycling on a road so much faster than cycling on a cycle path?
Given the same day and the same rider and otherwise equal conditions, riding on a tarmac road vs riding on a gravel cycle path, the road will be faster.

Why?

It's about the rolling resistance of the surface.

This page gives a good breakdown of the physics involved in working out how much power you need based on various inputs.

But basically, a rougher surface offers more rolling resistance to your bike tyres, so you need to put more power in to sustain the same speed. Given you're likely to put out a constant power (since we're assuming identical rider and conditions) on both track and road, since there is less resistance, you'll go faster on the road, the 'spare' power will come out as speed.

Given identical surface, then you have to look at what else is different in the route. There's a 0.2 mile difference in distance, but, what's the ascent and average gradient of both routes? Plot them on bikehike.co.uk (or the mapping tool of your choice) and see what the difference is.

A local route to me on the Trans-Penine Trail turns out to be a 2% gradient, though it looks flat, on gravel. The road route near by has some sharp hills, but has downhill segments and a much smoother surface, so is faster on my MTB and far faster on my road bike.

Also, on the road are could be benefiting from the drafting effect of cars possibly, whereas on the cycle trail you might be cutting the air yourself, that can account for a 20% energy saving.

And finally, for now, there's always the right of way, if you're having to slow on the cycle trail to negotiate slower cyclists or other route users, whereas on the road you're the slow thing people are moving round.