That phone has a SD855+ chipset which is pretty decent. But 4K video editing is a very CPU/GPU intensive process, so it makes sense that it will still take some time to process the videos.
Why don't you use a PC (with a video-encoding GPU) for the video editing? It will be far more efficient & faster than doing it on the phone.
If you must do it on a phone for whatever reason, then i guess you should aim for a phone with at least SD875/SD888 chipset. I don't know if you can find such a phone in your budget though. Visit the market & consider your options.