Being faster is a desirable quality in almost every sport. Many people mistakenly think speed is completely attributable to genetics and there’s little you can do to fix it. While that’s completely incorrect, we also need to recognize that to make big improvements in the physical, and sometimes the technical, capacities associated with speed is a gradual process that must be continued over time to expect maintenance or improvements.
Technical changes are a little less cut and dry. In some cases, you may change an athletes mechanics to what you perceive to be ideal only to find them initially slower as they learn to adjust to new positions, muscle activations, encountered internal and external forces, etc. And I’ve also had the rare case where I’ve changed things and even after a conservative adaptation period the athlete still didn’t improve so we had to revert back to the old ‘less efficient’ model.