How to easily upscale a 1080p video to 4k resolution using FFmpeg and a one-line command.
The speed of the video upscale will depend on the video length, bitrate, your CPU and what settings you use for the upscale process.
The FFmpeg command
Lanczos is a common scale algorithm to use, avoiding a blocky looking video after the upscale.
ffmpeg -i input.mp4 -vf scale=3840x2560:flags=lanczos -c:v libx264 -preset slow -crf 19 output.mp4
To achieve a faster upscale use a fast preset like veryfast or superfast however these will give a poorer compression but will complete the upscaling faster
ffmpeg -i input.mp4 -vf scale=3840x2560:flags=lanczos -c:v libx264 -preset veryfast -crf 25 output.mp4
Using a CRF of 25 will also lower the output quality slightly (less bitrate = less size).
You will have to find the perfect preset for how much time you want to spend upscaling and the output quality.
This is all in line with video encoding concepts, slower encoding means more quality is kept and the compression is superior. You cannot simply speed up the process or take shortcuts.