Every sentence contains content words and function words. Content words are the ones which carry the meaning and are considered important. It is advised to lay emphasis or stress on these content words. At most times last content word of the sentence recieves the stress (depending on the type of stress, emphatic, new information, etc one content word in every sentence recieves prominent stress).

Even the rise and fall of the tone (intonation) starts with this content word depending on the number of syllables and which syllable is recieving the stress in the particular content word.

Now my question is, how important is syllable stress of function words or content words not recieving stress, in fast speech?

How much will it impact intelligebility, when a person does not employ syllable stress in function words or words which are not stressed in a sentence??

I feel intelligebility is not impacted in fast speech, if it is taken care that proper syllable stress is employed in stressed words (content words).