How long one SimRun chunk may take. A second, not a tenth of one. The renderer does nothing while the sidecar simulates, so this is the granularity at which a progress bar moves and at which a Cancel is noticed - and a second is fine for both: a bar that steps once a second is a bar, and cancelling is rare enough that waiting out the chunk in progress costs nobody anything. Ten times a second bought no more of either, and cost ten times the messages.