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.
Snap packagers need to opt in to include lzo compression so that it's not using the slower xz compression when they're installed on your system:
Spotify is one of the worst cases of this and they probably should turn it on. In certain cases, like Chromium, the difference was significant and now it launches fast once that change was committed by the packagers.
I think the most simple answer is a bigger package will take more time to start than smaller ones, Spotify is based on electron which (I think) is essentially translating a web app into a package/app which is convenient for companies because they don't need to hire more developers to work on another version of the software but the other hand, it's taking too many resources, we can only hope that developers and companies make more effort into making their apps (especially paid ones), I'd recommend on upgrading to an SSD for better performance system-wide.
I can confirm that Atom and Slack Snaps are slow to launch. After replacing Slack Snap with a .deb package the app launches very fast.
I suspect both are Electron packages. The reason Electron app launches take extra time has to do with its architecture. Electron uses Node.js and Chromium as backend/frontend. When you launch an app it has to build this runtime environment first which takes time.