While you encounter an issue of conversion speed slowing down to 0 and then freezing without any progress, you can try these first:

1. Give Passthrough profiles a try and see if the process can go through or not. 

2. Go to Common Settings>General>A/V Codec, tick the "Disable all GPU codecs for decoding and encoding" box, and uncheck the option "Enable Lightning-Recoding when applicable (accelerate H.264 software decoding and encoding)" to convert again and see the result. 

3. If it doesn't help, please send feedback to the support team for troubleshooting.