snap
Straight from cold Fossa boot, until usable user interface:
Blender 2.91.2: 4 seconds
Gimp: 11 seconds
Boxy SVG: 22 seconds
Spotify: 43 seconds
The latter two seem like electron apps and take forever. Since these electron apps seem to be increasingly common, is there a way to speed up their snap load times? Spotify in particular is egregious.
Snap packages are squashfs file systems. The only way to access snaps is to mount them. So yes, they will always be mounted.
They are a different thing from what traditional package managers implement. I would say they are neither a replacement for .deb packages, nor are they complimentary.
Here is a long tabular comparison of AppImage vs. Snap vs. Flatpak features. It is from the AppImage Wiki on GitHub:
Note that this comparison is mainly from the perspective of AppImage, although it tries to represent each project fairly.
Best Answer
This is not a direct answer as it isn't a way to speed up snaps, but it may help anyway.
Spotify is available as a .deb which you can install in Ubuntu instead of using the snap.
I tested both options on my system and got the following approximate results:
Snap -> 7.5 seconds
Apt -> 0.5 seconds
Which raises another point. You mentioned in a comment that you're running on a 2.5gb/s NVME SSD which suggests to me you have a fairly modern PC. I'm running something similar but I can load Spotify as a snap in 7.5 seconds compared to your 43 seconds. Perhaps you have some other issue on your system. 43 seconds does seem excessively slow on a modern PC, even for a snap.