Upload Twitch bitrate changes a lot?

I wanted something important and I have a Fritzbox 7490. In the speed test I have 5mb / s upload. I wanted to start with Twitch and always get lags at 720p 30Fps. And have my bit rate on 1800 because that is the minimum requirement. I always get skipped frames because the 1800 are always sent, but often it goes down to 900 or 300, I get a lag in the stream and the bitrate goes back to 1800. And that happens often, I've tried a lot, but nothing has helped Wi-Fi and can't use Lan.

Try to reduce your processor load by default, for example from faster to fast