To return to the point of the OP, actually speed does matter when you are talking about simulating brains and what they do. Your brain does not exist in isolation. It is constantly taking in information, at high bandwidth, from the rest of the world and the rest of your body, and outputting (via muscles, glands, etc.). Furthermore, those outputs affect the inputs, and are often done specifically for that reason: every time you move your eyes (which you do several times per second) or scratch your knee, or sniff the air you are changing the inputs to your brain, usually because your brain is actively looking for certain environmental information (what is over there to the left? what chemicals are in the air?) that may be relevant to your ongoing behavioral purposes.
This continual, high bandwidth interaction between the brain and the environment and the rest of the body is essential to human cognition and consciousness. After all, that is why we have brains, to enable us to interact more adaptively with our environment (and to maintain bodily homeostasis). The trouble is that the world, and the body, runs on its own schedule. If the brain computes what it needs to compute too slowly, its output signals will be too late to be relevant. That smell will no longer be in the air (or the poison gas will already have killed you); the tiger whose movement you noticed in your peripheral vision will have eaten you before you are even able to look towards it to see that it is a tiger, let alone take any defensive measures; but, in any case, you won’t be able to see properly, because the iris of your eye will not be able to contract quickly enough to shut out the dazzle of bright light (or dilate quickly enough to let in sufficient light when you eventually turn your eye towards the shade where the tiger might be hiding).
In a way, though, the tiger is a misleading example, because the point is not just that a brain that works too slowly will get you killed (although it will), but that this high level of real-time interaction is constant (much of it below the level of consciousness, but essential to maintaining consciousness), and it is really what cognition in general, and consciousness in particular, are all about. Computation on its own (even if it is computation of the right form: even if the algorithms being computed are the one the brain actually computes) won’t get you consciousness.* Computation that controls rich, two-way interaction with the world probably will (indeed, probably does) get you consciousness, but if the computation is too slow the world will have moved on, and the interaction will break down.
*If, *per impossibile*, you could cut off someone's brain from all inputs and outputs, it may be that consciousness - thinking, remembering, imagination and dreaming - would continue for a short while. However, I am confident that the mind would soon fall apart. Try to imagine what it would be like: absolute nothingness. If you could somehow make a brain that had *never* had any input or output interaction with the rest of the world (note, even a foetal brain does have some degree of interaction) it would not be conscious in the first place. It would have nothing to be conscious *of*.