I have a Roku Express (3900eu) which works perfectly without any problems except when I'm mirroring.
When I'm streaming a movie, the audio and video will stutter every 10 seconds or so, only for a brief moment but its enough to make the movie unwatchable.
I've tried mirroring from 2 different laptops, a tablet and my galaxy s9. I have even tried different file formats, 4k video, 360p video, internet router turned off but the playback is consistently the same with the stuttering. Even placed the mirroring device right next to the Roku. Roku has been updated with the latest firmware.
I'm really not sure what else I could be missing that is causing this as most problems in the forum either have mirroring working fine or not working at all.