The most interesting thing to me is your impression of the Mezzo, way better than the Swift, right? That is really weird.

Other than that, probably the biggest 'factor' is bike 'feel'. Some bikes 'feel' just a lot better than others, and some of this 'feel' translates into actual numbers, but not all. The most readily called to mind 'feel factor' is tyre pressure. Even seasoned pros think that harder pressure tyres are faster than optimal pressure tyres, but tests show different. So you can't trust the feel 100%.

So my Swift 'feels' a whole lot faster than my Birdy (which is not that dissimilar to a Mezzo) but actual testing reveals only a small difference.

I rigid bike will feel a lot more responsive than a mushy one, but it ain't necessarily much faster. Likewise, a light bike will feel a lot faster than a heavy one, especially if the wheels are nice and light, but the overall picture may not be that different.