Burning subtitles into the video implies re-encoding. And if you want maintain video quality, you'll have to choose a pretty good quality setting like crf=24 and this will take a lot of time even on H.264/AVC.

You are looking at multiple hours (>2) for a 2-hour movie from my experience. One way to reduce this is to spin up a DigitalOcean droplet (a $80 per month droplet) that'll give you a lot of processing power, finish your encoding in 2-3 hours,and shut it down. It'll save you a lot of headache and time.

Not sure I answered your question, but, hope that helped!