I tried to start a fairly big application using a link in the 'Application Launcher' (AL), (made using the 'Edit Application Launcher...' in the AL context menu), but it was very odd.
When i launch the application using a filemanager, (Konqueror or Dolphin) it takes a few minutes to fully load.
When started with the link in the AL, it took over 10 minutes just to get through the first 10%.

Is there some trick to making 'good' AL-links? Or is this something to report to the developers? How do I troubleshoot this issue further?

Some more information about which application you are talking about would be helpful (mind you, your Plasma version and distribution, too).
All application are usually started with arguments you can fin by typing the application name followed by --help in a konsole

Mamarok wrote:your Plasma version and distribution, too).

Kubuntu 18.04 and plasma 5 I think
I'd need more help to identify my exact plasma version, but I've kept up with updates, so it would be safe to assume it is the newest release as of today (2020 March 21).

The Application in question is 'KSP.x86_64', the primary executable for 'Kerbal Space Program'. It is a Unity based program.
I don't think that what application is relevant, as it starts fine when launched via a file manager.
It might be because of a quirk where Unity is inefficient when started using a command with a long path, as is the case with the link I made for the AL.

But I don't know that, how to work around that if that is the case or how I would troubleshoot this issue further.

FWIW: KInfoCenter gives you all the system information necessary.

On the contrary, it really did matter to tell which application you are talking about, as it is not a KDE-made application, so might be a tad more tricky to solve.
I am not familiar with the Game Engine Unity, but I guess it needs to start quite a few other things, did you check the arguments used when you start KSP from the File manager?

Mamarok wrote:FWIW: KInfoCenter

Kubuntu 18.04, Kernal 5.3.0-4-generic, KDE Plasma 5.12.9, KDE Frameworks 5.44.0

About arguments; I don't see any arguments in the 'KSP.x86_64's properties. It is identified as an 'executable', and has been granted permission to be run as executable.
Are there any general arguments I should be on the look-out for? If I wanted to start the application as 'High Priority' (without having to check in as a Super User), what would be the argument or arguments to use?
And how would I add those arguments to either the executable in the file manager?
For the AL, there is the command-path, for which I assume arguments can be suffixed. Would those arguments go inside or outside of the 'PathInQuotes'-quote marks?

Solved:
The issue was that the Command path was fairly long, but work path was not set. Adding it removed the problems.