Thank you, jrmichler! If there is no other way, I guess I'll have to live with longer simulation times or try some other tricks like leaving some computations for only every n-th time step.
Regarding the drag coefficient: I got the value 0.8 for a Reynolds number of 201.5 from figure 4 in
P...
I am working on simulating the movement of small spherical objects under the influence of drag and a number of other forces that, at least for now, don't depend on the object's velocity.
Normally I would sum up all these forces, including drag, to ##F_\Sigma## and update the object's speed...