Setup with Ubuntu and Focusrite Scarlett 2i2 - Low Performance?

Set the audio priority at 95 in qjacktcl.
Turn the wifi off.
Try again.

Keep in mind that using jack is needed only if you want to connect applications to each other.
If you juste want to play Pianoteq, you can use Alsa direct without jack.

Can you give us some informations about your pc?

Well, I'm not having anything better at the moment, and I still want to make it work as good as possible. Note that Pianoteq alone does run fine on low Settings (Soundquality that is fine for me 44100 Hz sampling rate, Buffersize of 192 samples, no noticable latency), if I use it via Alsa. I was just wondering wether Jack could do a better job with the external audio device, and I have to get it work anyways if I want to record stuff with Ardour. 

Can you explain that in more detail? If the sound is created in the computer, what exactly is the soundcard good for? Is my assumption right that for virtual instruments, the choice of the soundcard is irrellevant?

The soundcard will give a better sound with less jitter and parasit noise + the ability to have more inputs and output.
If you just want 1 headphones output and if you are happy with the onboard soundcard, stay with it.

Ubuntu Studio is solid, it should run fine.

1. Click qjackctl
2. select the soundcard you want to use
3. set the desired sample rate and buffer size
4. check "realtime"
5. set the audio priority to 95
6. select the right power plan: you should use "performance" instead of "ondemand" or the other plans.

7. start Jack using the button on qjackctl

8. start Pianoteq and choose the auto connection to jack.

Et voilà

Do I understand right the the Buffer Size tells me how many samples are processed before they are sent to the Soundcard? If it is like that, then why are XRuns more probable if the buffer size is lower?

This is probably getting to the core of my non-understanding. I will state my line of thoughts, and I would appreciate it if you tell me for each of them individually if it's wrong, in case it is wrong:

Let's say we have a sample rate of 48kHz. And a Buffer Size of 256 Samples.

1. Sample Rate of 48 kHz means that the sound card expects 48k samples per second, which it processes to audible signals.
2. This also means it expects 48 k / 256 transmissions per second from the pc.
3. If the computer fails to deliver the 256 at the point where they are expected, that results in an xrun.
4. This means the Computer has to provide 256 samples at every transmission.
5. The computer still has to compute 48 k samples per second, independent from the buffer size.

So where is the samplesize a factor that makes xruns more probable? The computer doesn't have more work to do when the buffer size is smaller.

I guess "one Block" means 256 samples, in our example? (This question is about terminology that I'm not yet familiar with).

I agree with you then, the smaller the Block Size, the smaller is the time that the computer has to calculate the desired samples. This is a linear relation.
But: The smaller the Block size, the smaller the number of calculations that have to be performed, right? (In order to process 128 samples, the computer has to do half the work compared to processing 256 samples).

To Stick to that example (and following my line of thought), I'd counter that it's as difficult to build one house in 30 days, as building 1/10 House in 3 days.

Thank you for this definite explanation.

If I have half the data to process, but I also have to do it twice as fast (which means in half the time), shouldn't those effects cancel out?

Ah ... I think what you're getting at. It's because of the way the CPU works that the time required for processing, the amount of to process data and the effort are not strictly proportional / antiproportional?

To use another quit simple example: It takes at least 2 hours to bake 16 little breads, because I have to make the dough, let them rise, and afterwards bake them for 1 hour.
This time won't change if I bake 8 breads. It still takes around the same time for the to rise and to bake, and a little bit less time for me to make the dough.
Then I'd get it.

This I understand, it's absolutely logical to me.