Can't add much except on the GUI: for Qt apps like Calibre, loading the Qt libraries and rendering the GUI also takes its time. Though that too is becoming negligible on modern SSD systems. One of my Qt apps, if cached, is oft ready at 150ms, but the UI still takes over a second. Fast enough.
Some factors are hard limits, or not under a developers control. Or can't be easily changed or optimized. Aside, and if wanting to be mean, one could argue the Citadel UI (Svelte) is web based, requiring a browser, an even more complex beast
