What is the normal rendering time? I have i3-7100 CPU and 16 gigabytes of DDR4 RAM. I know it's not much, but why Adobe Media Encoder 2020 is rendering my simple 720p video with subtitles so slow?!

It takes 20 minutes to render my 5 minute 720p video with default H264 settings. What is going on here?

And there are no scaling or other effects. Just the video and hardcoded subtitles.
Check your GPU settings in AME prefs should be targeting your particular GPU. Choice of GPU can effect render time significantly. A GPU with a high CUDA processor count is best for these renders on a PC. On a MAC it should be metal with an appropriate card. Try and get a view of your system performance and buy a better card. https://www.cgdirector.com/best-cpu-gpu-render-benchmarks/