Were dinosaurs slow movers?

It depends on relative speed of predator and prey. If you imagine a t-rex, it is very big and very heavy. The legs have to take weight of the body at every stride and so they have to be strong enough to support them. Their legs were enormous and we can estimate how strong they were. It has shown that it is unlikely that they would have been able to run very fast: maybe about 20 miles per hour. That's a t-rex. The smaller light things like ostrich dinos, are light and long-limbed. They have the physical construction that would have allowed them to run faster. So it's all to do with scale. Smaller, lightly-built animals would have been able to run faster as long as their lungs were built in the appropriate way.