hey guys, you are not understanding us, we are not
  understanding what you say as well, and it is hard to find
  people that, once your understanding limits are obvious,
  will try to slow down the peace of the conversation. Often
  even if I say I did not understood, I'll get the same
  sentence repeated the same at speed of light.

This is an important point. There are a number of things that native speakers of English can do to be more understandable to non-native speakers. Some things I've found are helpful: speaking more slowly, shifting your pronunciation (especially of vowels) to be more standard, using different words (e.g. if you're talking to a native speaker of a Romance language, prefer words with Latin roots over Germanic ones), and being careful to pronounce all consonants. Things that rarely help: speaking more loudly and repeating the same phrase multiple times.

Making these changes to your speech in real time is a difficult skill that must be cultivated and practiced. I would encourage native speakers to work at this skill, as it can make a big difference. I was once speaking to a couple of Russians and an American from the same region of the US as me. The Russians said they could understand me easily, but had difficulty understanding the other American. The difference was that I was making an effort to speak more clearly for them.