Uphill Momentum?

One distinct aspect of ride quality I've been noticing in different bicycles, is the way they retain (or fail to retain) momentum* when going uphill. I am pretty sure that this is a separate issue from the way a bicycle climbs hills in general, though I haven't found any literature addressing it specifically. But in any case, what I mean is this: Say you're cycling super-fast, either downhill or along a flat stretch, then suddenly in front of you is an uphill stretch and you take it at full speed. Initially the speed you've already attained will propel you, and only once you've spent that momentum will you need to switch into a lower gear or pedal harder. The point at which you lose the momentum depends of course on how much of it there was to begin with, as well as on how long and how steep the hill you are climbing is. But in my experience, it also depends on the bicycle - with some bicycles being better at it than others, aerodynamics and weight notwithstanding.

Take for instance, my formerly owned Pashley versus my current Gazelle - both of which are heavy, upright loop frame bicycles made of hi-ten steel. The Pashley seemed almost indifferent to momentum: It was not much easier on climbs after picking up considerable speed on a descent, than it was after starting from a complete stop. I could be flying downhill at 20mph, but as soon as I'd hit an uphill stretch, the bicycle would just stop dead. The Gazelle, on the other hand, retains momentum surprisingly well - soaring uphill if I manage to pick up sufficient speed. I have sensed this difference to various degrees in a variety of other bikes, and I want to make it clear that I am comparing like to like: roadbikes to roadbikes, upright bikes to upright bikes - so it is not a matter of aerodynamic advantage or weight.

Though I am fairly certain that what I am experiencing in this regard is real, I am not sure what accounts for it.  If it's in the geometry, then it must be something fairly subtle - as I am sensing a difference in bicycles that, in principle, ought to handle similarly. Have others noticed what I am describing? What do you make of it?

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* I am using the term "momentum" here loosely and colloquially. From the point of view of physics, if all of my bicycles start riding up a hill at a particular speed, they will reach approximately the same point before they stop, slightly variable due to rolling resistance, weight distribution and aerodynamics. So, I am not using the term as it is used in physics (i.e. conservation of momentum, energy, etc.), but rather as shorthand to describe how each bicycle behaves while being pedaled uphill under my power after having first picked up speed.