Why are some snaps fast, and others so slow?

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.

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.

You are correct: Some cross-platform application development toolkits --like Electron or Mono-- can offer high compatibility, but also can cost a price in performance. As a user, there is nothing you can do about it at runtime.

An application packaged using Snapd should not run noticeably faster or slower slower than the same application packaged another way. If you can reliably reproduce a significant difference, then please file a bug or issue with the snap author, AND please raise the issue in the forum at snapcraft.io.