Thank you once again for your help
If I understand it right, the particles should be calculated by 5-6 CPUs then? If so, the problem that slows down my simulation has to be somewhere else...

I do understand that I need small particle time step size at a size of 1/20 of the calculated collision time (due to the max. allowed overlap of 10%). Also I agree that I need to run the simulation longer to get those collisions. Problem here is the endless simulation time (with my approach) which prevented me from achieving a converged behaviour.
If the number of CPUs computing the DPM Iterations overall is not responsible for the slow down, my guess would be that the collisions that occure even before reaching the collision wall are the reason why it gets so slow. You can see them in red for the top injection point

The collisions there occure because the drag influence on the first particles is higher than on the later ones -> if I simulate long enough to achieve some sort of "steady" state, this should not occure anymore either I guess.

Thanks to your Input, I got an idea and tried following:
1.) Only simulate the fluid until it is converged (100 Fluid iterations).
2.) Turn OFF DEM Collision Model
3.) Particle Time Step Size: 1e-6s, No of Particle Time Steps: 10 000, DPM Update Interval: 1, Number of (Fluid) Iterations: 10 -> Total (physical) simulation time: 0,1sec
To achieve the "steady" behaviour but to not slow the simulation down through collisions where I do not want them.
4.) Write Data
(this took only 2 hours to simulate)

5.) Load that Data and turn on DEM Collision Model
6.) Particle Time Step Size 1e-8s; No of Particle Time Steps: 10 000, DPM Update Interval: 1
7.) With already particles distributed in the domain, simulate only few Iterations to get the collisions at the collision wall/rebouncing particles.
Like this, it should not take so long anymore.

First question: Is there any mistake in this "plan" or could it be a solution to work faster?

However, I am having trouble executing that plan. I got to Step 4.) but there occures a problem preventing me from proceeding with the DEM Collision model.
Here you can see a screenshot I took after step 4.)
As you can see, the "blue" injection (Injection-9) got to slow and for that reason did not reach the boundary.
Instead of being aborted after some time, all of those particles remained in the domain for the entire simulation.
Without them, I would roughly have 1500 particles in Fluid at a time. A number that should be reasonable to use the DEM Collision Model.
Problem is, that I do not get those particles to disappear, even when changing tracking parameters.
The tracking parameter where on "default" for this first try -> Tracking Parameters -> Max. Number of Steps: 500 and Tracking Parameters-> Step Length Factor: 5.

I restarted the simulation from Step 2.) with these settings:

However, still all particles remained in the fluid. I only simulated 10 (fluid) iterations to get results faster, but as you can see it did not work.

This brings me to the second question:
How do the tracking parameters interact with the other Parameters (Particle Time Step Size, No of Particle Time Steps, DPM Iteration Interval, (Fluid)Iterations)?

From what I read in the User Guide the particles should have been aborted after 5 Particle Time steps. In my simulation it did not abort any particles though.