Where is plyometrics? My understanding is they are key to speed, yes you need to be lean yes need to be strong, yes mobility will help, as will working on technique (Both biomechanics and actual practice), but you also need to have very stretchy tendons (e.g. the Achilles tendon, also patellar tendon) because though the muscles generate the force the tendon acts as the spring. E.g. we all know someone who is strong but not quick, They might even be strong and lean but again not the quickest, and that will probably will be because of their inelastic tendons. Likewise you can know someone who isn’t strong in an absolute sense (though they will be strong relative to their body weight) who can be very quick, and that will be down to bouncy tendons.