There is temporary solution for 1.4.
Codec x264 and probably libvpx have option "threads" already.
To overwrite default settings MLT for x264 you can use ffmpeg's -vpre swich and x264 options storage will be .ffpreset file.
To do this change 2 files in openshot sources: video.py and render_cli.py,
if self.render_options["vcodec"] == "libx264":
erase all existing options for x264
and add 1 line:
and recompile openshot.
all options for x264 are now will be stored in ffmpeg .ffpreset and mlt allow to overwrite its default settings. file looks like this and is equivalent "veryfast" x264 preset :
Now we have editable file , and we can change presets from ultrafast to placebo /68 times more time consuming than ultrafast/, and can change it to various sort of devices . Someone complains that x264 is time consuming. With presets is flexible enough.
I also add option crf which do me Constant Quality of the rendering picture and somehow switch off bitrate setting from openshot, and do me benefits of using veryfast or ultrafast settings in x264 do not wasting any bit set of quality as regards to bitrate.
In this way 1 minute of 720p film rendered 3 times faster then before on my 2 core processor or 5 times in ultrafast preset with the same quality.
By the way
New ffmpeg allow using x264's "preset" swich directly but mlt 0.7.4 somehow does not allow change its default settings in this way.