Paralleling many of the other comments that give excellent test advice, it's also worth seeing if your CPU has Intel Quicksync support, which can give a hardware-accelerated rendering option to ffmpeg. See below link for reference of the flags.

https://trac.ffmpeg.org/wiki/Hardware/QuickSync

Even the most low-end processors like the Celeron, if they have embeded GPUs, have this encoding acceleration built in and it makes a huge difference versus using the general purpose CPU portion. The generation of CPU will determine the encoders/decoders available.

I learned about this from the below post, and have been using the functionality where ever I can as it applies many of the popular transcoding sofwares out there, for example handbrake also.

https://forums.serverbuilds.net/t/guide-hardware-transcoding...