For resistance training outcomes, rep speed generally does not significantly affect muscle strength or growth, as the primary factor is training to muscular failure.
However, slowing down repetitions offers an indirect benefit by reducing mechanical tension peaks, which lessens stress on joints and connective tissues.
This can lower the risk of injury and improve long-term recovery, especially if you experience aches or are recovering from an injury.
If adopting slow reps (e.g., 5 seconds up/5 seconds down), you must reduce the load to maintain proper form and still ensure you train until muscular failure.