It depends. If the blocking I’m having is related to stress and indecisiveness because I’m speaking faster than I can organize my thoughts, then slowing down would help. But if the block is due to a particular word I fear stuttering on, then speaking slower might not help. However speaking slower does always provide an opportunity to change your thought process to something that frees up your mind, or provides positivity. It can also provide you more time to deep breath. These things can help fluency.