I can't understand people that speak fast.

Most of the time, when learners can’t understand fast speech it’s because they haven't acquired a lot of the vocabulary that’s being used. If you can understand a specific sentence only if it's written down or spoken slowly, it probably means that you have conscious knowledge of some of the words, but you haven't fully acquired them yet. Your brain is doing a lot of work translating words and trying to recall their meaning, but that can't be done fast enough to keep up with language spoken at a natural speed. By watching a lot of easier videos, you’ll have enough comprehension to continue acquiring those words and tie their sound to their concepts directly. After doing that, your brain won't need to make any effort to understand them, and you'll understand them even when people speak at full speed. There is no need to practice listening to people speaking fast to be able to understand fast speech.