(I know Kemper does not have such a gap at all.), But... Helix always has had a gap when switching presets. So does the Boss processor and most other old-school digital processors. On the Helix, you have snapshots to circumvent that gap, that (annoyingly) is there. Each snapshot is your different sound within a preset, which is your virtual guitar rig.
Preset spillover does not yet exist in the Stadium, as I understand. On the Helix, you sacrifice one DSP chip, to be used as a buffer for switching your presets. So you sacrifice half of your signal chain, and have pretty seamless switching between your presets. I never found myself in a situation to have needed this "feature".
On the Helix Floor, I actively use snapshots and do not ever have a need or reason to switch presets during a song. Each preset is each own song. There's a gap when switching, but that that's when you stop and start the next song. That's how modellers should be used -- each preset corresponds to each song. Otherwise, you have several presets that can cover all your repertoire, have a preset for each type of song you'll ever need.
In my set, when I was touring, I had 26 presets, which I went through on a nightly basis. I organized them in the order of the set, and just pressed "up" to go to the next song. We had 2 songs that were connected, and I just found a "gap", where I could switch to the next preset. But trust me, once you start creating individual presets (which is what I do), there's no going back IMO.....