As an example, and admittedly in a completely separated energy range: neutrons emitted by uranium have to be slowed down in order to keep the reaction going at

As an example, and admittedly in a completely separated energy range: neutrons emitted by uranium have to be slowed down in order to keep the reaction going at the desired speed. If you don't slow them down, they'll just traverse other uranium nuclei and the reaction will only proceed with its natural speed (half-life of billion of years). That's why you need a minimum mass of uranium for a nuclear explosion: you need neutrons to traverse a certain thickness of material before it gets slow enough to excite other nuclei