It seems to me that you paid a lot of attention to your speech at that moment. I don't know if when you spoke slowly, you were thinking about the words accompanied by some feeling. The issue of words that don't come out, for example, those starting with "S," is really like that? If you tried pronouncing the vowels and consonants, you might notice that you can pronounce them well (fluently), for example, the "S." Now, what is the problem? Why do we find it difficult to say certain "SPECIFIC" words, for instance, those starting with "S"? I think it's the value we give to those specific words. For instance, if, before asking someone "what time is it?" I accompany it with some feeling of fear or distrust that I won't say it correctly, or that it contains a letter that "I can't say," I clearly won't say it fluently or, in the worst case, I might get stuck. In essence, all of this is closely related to how stuttering occurs. Anyway, I don't want to write too much, or it would become very long. I hope my thoughts help you a bit.