Cekuhnen, there is a lag (In windows, too) when starting a stroke, and depending on case, it can do a straight line. I reported several issues with the brush engine, long ago, Mark told me they would probably rewrite the brush engine. There have been, though, improvements here and there, but several relevant issues do persist. (I made very long, screenshots filled threads for documenting the issues, even some video, for both AP and AD, in beta times and a bit after that, too.) I'm drawing with Krita in the meantime, but keeping a very close eye on progress here, (as I am interested in both, Krita and Affinity).
Other issues were in its day that if you did draw zoomed out, it could cause jittered (heavily) lines, but it was solved to a big degree with an update, now I don't know the status, to be sincere. There was an overall lag (very noticeable in hand writing), and the lag starting the stroke. Also, the continuous refresh/redraw of the screen even only moving the cursor -not necessarily drawing- as is how the engine works to optimize performance (this can be distracting for someone drawing). The redraw would not happen if you draw zoomed in (ie, for details) or just doing pixel art, as then you are always "zoomed in", so. There was jitter when you draw (more than with other apps), in Photo. Lastly (of what I remember that I reported) , aliasing. To me this was not an issue, as I did certain setting to the max, and it made a sort of smooth line. For some people this was the same issue (although I never understood why people did not apply my trick to that. ). In AD, I reported in detail that when you paint with AD, the raster preview is correct in position, but it then (when you release the pen pressure, lift the pen from the tablet) "vectorizes" it and as a sort of jitter in the process, very noticeable, another user, an illustrator, had reported the same, I just went into detail. This might have been solved, too. I believe a fix was made, but it kept happening in certain degree in which I cannot use it (for free hand painting, but as I said, both applications are very useful anyway, despite the brush problems. ).
Please, consider that a 90% (or not) of that might have been fixed already. I just am working with other tools, for the moment (but watching the updates lists from time to time). I explain it here to you just in case you notice some of that, is not your side, your system, machine or Wacom. The good news is that at some point they might rewrite the brush system. Not in all machines/systems happen every issue. Hardware has an influence here. I believe the issue is in all, it's just that really good hardware can make it better in some problems.
(always to help a Blenderhead ;) )
That said, 1.6 is around the corner, with a line smoothing feature. I wouldn't be surprised if those issues, maybe all, are solved in this update. We will see. :) I purchased AD, as I find it (VERY, VERY) useful in my work flows (AP would be too, in all its editing features. But I have those covered in several ways, for now) even using only the mouse, but refrained to purchase AP until the brush matters are fixed... So, might jump in the wagon with 1.6. :)
PD: AP --> How we call here Affinity Photo, AD --> Affinity Designer.
Lastly, I believe Affinity tools are going to be really a big thing (way more than they are already). That is why I am so interested. They are in a continuous improvement process, and I think it's really worth it. Even more when I just read Adobe CC subscriptions are going to be raised in a whooping 11% to 18% in March...Yikes ! (indeed, I had totally predicted that, some fellows friends of the profession did not believe me... Ha ! ) Affinity, please, survive.... ;-)