When I change to a lower resolution, playback is stuttery

Connected my 2 new LG 4K displays to my 5.1 via Display Port (RadeonRX Vega 56). Playback is running smooth in native 4K display resolution. When i change to a lower resolution, which is needed because of too small letters in 4K, the playback is getting choppy. Changing the the same resolution (no matter which) to a "low mode", holding the option key while right clicking, the playback gets smooth again. I absolutely don't understand, why it works in full resolution, but not in lower resolutions. Tried everything (including setting the audio input to "none). I hope, someone has a solution, because i don't want to work with a lower resolution setting, which results in a blurry image. Please, Adobe.. work this out!