Since this is a simulation on a serial machine, the cells can only be updated sequentially. We use a standard technique for avoiding weird effects due to the possible ordering up the updates of the states of the simulation components (the cells) by double-buffering the cells. That means that at each time step, the cells inputs come from the outputs of the cells at the previous time step. The new outputs of all the cells are computed but are not used until after all the cells' new states are computed. Then, in a separate iteration over all the cells, all the cells output are updated to the ones that were just computed.
This double-buffering means that any changes need to be propogated through all levels of the simulation. A change to the stimulus will appear at the output of a photoreceptor after one simulation cycle, but will only appear at a cortical cell after approximately 5 cycles.
You can set the delay of the the simulation thread between iterations, and the number of complete iterations over all the cells during each simulation step using the Simulation/Properties... dialog.
A short delay will cause more simulation steps to be computed per unit real time. A large number of iterations will cause signals to propogate through the layers of cells more quickly.
The peak spike rate is determined by the simulation delay, because at most a single spike can be emitted during each simulation step. If you set the delay to 100ms, the maximum spike rate is limited to 10Hz.