Others gave you the general answer, but in OPs line of work they just manually rewrite and tune all of the core algorithms a dozen times for different CPU arch

Others gave you the general answer, but in OPs line of work they just manually rewrite and tune all of the core algorithms a dozen times for different CPU architectures and dispatch to the most suitable one at runtime. I don't have a link to hand but IIRC they go a step beyond dispatching based on CPU features, and dispatch different code paths for CPUs with the same features but significantly different instruction costs. RADs codecs are expensive but that's the expertise you're paying for.