[Development] How to speed up QML for KDE5 without QML compiler

[Ulf Hermann]

> Can you explain that a little more.
> Loaders trade startup delay for runtime cost.
> What are the units of measure for runtime cost?

Runtime cost is measured in CPU time spent while interacting with the application. In some places, such as when scrolling, increasing the CPU time spent can be problematic.

If you don't use a loader all the items you need are created when the application starts. That takes some time, but later when you use them they are already there and it takes very little time to show them or otherwise interact with them.

If you use a loader, the application starts more quickly but the items are created while you interact with it. This can lead to some unattractive effects like unfinished UI elements being visible and the frame rate dropping.

You can try some funny adventures I had with that by loading a large trace into the QML profiler, expanding all the categories and then comparing the vertical scrolling between
a, QtCreator 3.3
b, https://codereview.qt-project.org/#/c/103618/ and https://codereview.qt-project.org/#/c/103617/
c, https://codereview.qt-project.org/#/c/103648
d, QtCreator 3.5

I ended up writing a "SynchronousReloader" in order to be able to use asynchronous Loaders without having blank labels being visible during scrolling. However, I'm still not sure if the added CPU time spent on loading them after expanding the categories (and potentially during scrolling) is actually worth the reduced "startup" time when loading a trace.

-- 
Ulf Hermann