Yes, the random animation feature works as you pointed. The "Samples" parameter, defines the number of mesh samples, and "Time Offset", the difference in time between them.
For example, using Samples=4, Time=10 and rendering the frame 0, Forest gets 4 samples at time 0, 10, 20, 30. Rendering the frame 5, it will use samples at 5, 15, 25, 35.
The sample mesh generated at each time depends of the object used as source. TreeStorm creates a continuous animation, so all frames are different. Instead, a proxy has a limited number of samples used in a defined sequence (loop, ping-pong...). Forest determines the adequate frame sequence automatically based on the type of object.
In your case, the animation takes a long time to render (more exactly, in the pre-processing phase) because TreeStorm is very slow generating the mesh samples.
You can speed up it creating animated proxies from TreeStorm, and using them in Forest. The performance improvement is huge, because Forest doesn't need to request the animation samples to TreeStorm, since they are already stored in disk.
Are you using Mental Ray or VRay ?. If you need help creating the animated proxies, please tell me.