I consider myself almost fluent in English, but have trouble understanding when the words are blended together. This includes for example noisy places, song lyrics, or accents. In Czech, I have no such problems. I'm also learning French, where I can't see how native speakers can distinguish between the various contracted forms that make more words sound like one. I wonder if there is some fundamental difference between languages (for example, I'd speculate that languages with big differences between spelling and pronounciation would be less legible). Note that I'm talking about native speakers, not second languages (where the amount of experience is different).
First, let's consider noisy environment as barely equal to fast speech.
I guess this is related to several variables:
- Differences between words – the more different words are, the more likely you are able to correct mishearings. I don't see a simple metric here. Sometimes, there are pairs of two similarly sounding words that are unlikely to be confused, as just ome of them makes sense in such context.
- Typical speech speed – the slower people are speaking, the more likely you are able to correct mishearings.
- Word length – the longer words, the larger differences between words can be.
If we wanted to make an artificial language we could optimize all those three variables to some point. After reaching the point, you cannot optimize one of them without sacrifying some other of them:
- You can increase differences between words, but you need to increase their length.
- You can increase typical speech speed, but you need more significant differences between words.
- You can decrease word length at cost of sacrificing differences between words.
After all, any language with great redundancy is likely to be spoken fast (because they can) and any language with small redundancy is likely to be spoken slowly (because they need to). Imagine you speak your native language as fast as you can, so others still understand it well. It doesn't really matter what you language is; in any case, the ability of correction of mishearings will be about the same.
You might think than dialects of Ostrava (people speek really fast) and Prague (people speak really slow) are counter-examples. This might be the case when people from those two areas meet. But people from both areas have their own ability to handle fast speech.
I don't think there is any relation to writing system. A native language is both evolved and learned in the spoken form in the first place. Written form more or less follows the spoken form, and it is usually more or less behind. (English – written form is much behind of the spoken form; Czech – written form is slightly behind, see differences between I and Y or Ú and Ů or transition from W to V.)