Is there a way to set the maximum speed of particles? Using Newtonian physics I'm generally liking the way things are looking, however particles just keep on accelerating to undesirable speeds, even using "drag" and "damping" it starts off well but things ultimately get too fast.
There is no way to set limits on their speed by external forces.
ea at some time it will pass such point and thus the effect will be gone.
Though you can enable drag as a self effect.

In the particle system; go to force field settings
enable self effect
set an amount for example 16
set linear drag 1
set quadratic drag 0