1 - The layout algorithms should be improved to reduce their complexity. ( maybe I should implement a "true" force based, I mean that I translated the D3 algorithm, but thinking of it now, maybe it would have been better to take a real algorithm from a paper and to implement it from scratch ).
It depends. You need to have a sophisticated algorithm (probably based on quadtree as done in D3) to have it fast.
2 - Even if I'm not playing with the mouse, when the visualization is the active window then the computer turns. My CPU runs mad when the visualization is active in Pharo, and that must not be. There is no reason for your computer to turn if the visualization is already drawn and you don't do anything on it.
Yes, you spotted a nice problem. We are aware of it but haven't found a nice solution to it. A smarter way to generate event is indeed needed.
We left some todos from our last meeting, were you able to make progress on them? If yes, then you should focus on using Athens Matrixes. This is on our todo list for a long time. Using Athens may speed up Roassal, but I doubt it will make Roassal scale up to many more nodes. Who knows....
Cheers, Alexandre