You're going about things wrong on your computer. If you want to split a video file efficiently, you need to make sure you're not re-encoding the video/audio data. This can be done pretty easily with ffmpeg. For example, the following command will split a video starting at 15 seconds from the beginning, with a duration of one minute:
Code:
ffmpeg -i original.mp4 -ss 00:15 -t 01:00 -codec copy trimmed.mp4
This will use practically no CPU/GPU time! It's just copying data from one file to another, as it should be.
Nokia Video Trimmer effectively does the same thing on your phone. It seems to me like it works pretty well.