Hi @Bubo,
I do not have a solution for your issues. Rather I am also constantly reaching the limits of my hardware. Having said that: I am pretty sure that Sam has done everything to achieve the best performance possible as the first version of Sonic Pi was supposed to run on a Rasberry Pi. You might want to have a look at a thread I started.
Crackling sound, I am sure you know that, indicats that you are having Xruns, which is caused either by your soundcard and/or your CPU being not able to process the audio data in time.
- Are you using an external audio interface? That is an obvious way to improve your system’s performance.
- At least for linux (running the Jack Audio Server) it is recommended to deactivate Wifi because this can cause Xruns as well (the reason is: you need a system which works more or less in real time so any system activity, which is not necessary for your audio or the general os activity should be suspended)
- I did some testing (and a live performance) where I used external plugins and Carla (e. g. a reverb) which can be a better solution than running effects within SP (another reason might be, that you want to use effects or need certain characteristics of effects you currently do not find in SP)
At some point you will definitely run into performance issues (well, you did already 
), which has not neccessarily to do with Sonic Pi as such but the fact that processing audio needs time and resources depending on the complexity of what you are trying to do. At a certain point this would also be the case with Ableton or some other DAW.
At the moment I am also looking for a solution because I would like to have much more room (performance-wise) for more complexity in my tracks and/or my live coding. E. g. I just can’t imagine to run some visualization software (like Visor) because I need the resources of my laptop for the music rather than limiting what I can do musically by some additional software running. Probably I will have to buy faster hardware…