halacha - Which parts of the prayer service must be said clearly out loud?

I've heard a few different opinions on this. During longer prayer services, such as Shacharit on Shabbat, at least in Nusach Sefard, Pesukei D'Zimrah is quite long and there is a tendency for the chazzan where I daven to pray ridiculously fast. I'm fairly proficient at Hebrew, but there is literally no way that I could possibly enunciate every word clearly at that speed (and I have serious doubts that the chazan can either), let alone try to keep up.

I have heard from a friendly source that it's not completely necessary to say each and every word during certain parts of the prayer service, but I can't vouch for the correctness of this. I do know for sure that during specific parts of the service, extreme care must be taken to enunciate each and every word clearly and separately, namely the Shema.

Specifically, I have two questions:

1. Are there parts of the prayer service where it isn't strictly necessary to pronounce or say each word, and if so, what are they?


2. If one simply sits through the prayer service, says "amen" to the blessings when they come (baruch she'amar etc.), says his Shema clearly out loud, and pronounces each word of the "silent" Shemoneh Esrei, has he fulfilled the mitzvah of tefillah?

I'm looking for specifics here, and a tertiary question I might offer is whether this all applies when one is praying alone. I know that whenever I end up praying alone and I'm not being rushed, I make my best effort to pray every single word in the siddur clearly.