Why is spoken language so hard to understand?

As an L2 German speaker living in Germany, I have witnessed how slight vowel length differences matter. In my native Polish, vowel length is not phonemic - unlike in German. I've recently had a situation when I pronounced the word Krokette with a long E /krokeːtə/ instead of a short E /krokɛtə/, and I had to repeat myself a few times before the seller understood what I meant. I pay a lot of attention to properly timing my words, because this seems to be the biggest barrier for others to understand me in here.

One other thing is stress placement; Polish places its primary stress on the penultimate syllabe of each word, making it probably the only Slavic language without variable stress (sentence stress is an another case). Whenever I hear Czech or Ukrainian - closely related languages - I need some extra time to post-process what has been said, and then I recognize the words that are understandable to me to a significant extent. I'm usually not able to process those in real time.