By "slo-mo mode without lag" are you referring to the fact that when the simulation is running slowly, the user interface (cursor, drawing particles, HUD, element menus) runs just as slowly?

I suppose slow motion could be added. But making the user interface not lag when the particle update is taking a long time would be difficult, because of the need for synchronisation for most actions (e.g. drawing particles can't be done while the particle update function is running).

In most cases, just pausing the game then advancing one frame at a time by pressing 'f' works well enough.


As for going faster:
Particles update (move, and react with adjacent particles) once per frame. The speed at which the game runs is limited by how fast your computer runs the particle update function. The game cannot run any faster - there's no way to tell what the simulation will be like in 10 frames without running the particle update function 10 times.

The frame rate (displayed in the top left corner of the game "FPS: 42") is capped at 60 fps by default. So unless it's running at 60 fps (in which case you can make it go faster by raising the cap, as described in other posts in this thread), it can't go any faster except on a better computer.