seems a bit odd my phone can hardware decode x265 files has a snapdragon 765g cpu and that will play 40gb + sized 4k blue ray rips happily if they are stored on the internal storage in kodi vlc & mx player if i try to play the same files over wifi they get a bit choppy but that is to be expected.
When you say "decode", I think you're talking about "transcoding". Transcoding means "converting" video file formats on the fly. Phones don't transcode very well.
If the video file is in a format the phone understands natively, there's no need to transcode. In that case, when using a video file format that's native to the phone, the server is simply spooling the video file to the device. (No conversion required.)
If the user is experiencing problems with spooling video files, either something is wrong with the wireless connection between devices or there's a source of radio interference in the car. Perhaps a place to start testing is indoors, before moving the devices to a car.