My guess is these "waste products" are the free radicals that result
because of aerobic metabolism and the cellular damage they impose.
However, I do not know how long free radicals are present following
an intense workout, so maybe someone more knowledgeable on this topic
can enlighten us!

On the other hand, delayed onset muscle soreness is the direct result
of microtears in the muscle fibers and the inflammatory response that
follows (histamine, prostaglandins, potassium, leakage of plasma
proteins, edema, etc.). Having at least one easy day for every hard
day seems most logical to allow the body to heal and repair itself to
achieve a homeostatic balance.

:) Camille