[HELP] Output to mp4/x264 very slow

[Selur]

interlaced=False, interlacedd=False

first specifies input, the other output interlaced handling

Is there something I can do to speed the encoder up? It seems to get to 100%, then starts again. So far it has taken over 24 hours, now it says there's another 62 hours remaining.

You didn't share your encoding settings, but you want advice on your encoding settings.
If you used for example 2-pass encoding, this could be normal seeing your script.
Since your aim for YUV420P8 output, enabling 'Filtering->Vapoursynth->Misc->Source->Convert to target color space after source filter' might speed up the Vapoursynth script.

Cu Selur