x264 has a few generic encoding presets you can use when rendering. You can see the list of presets and exactly what encoding options they specify by running x264 --fullhelp. You'll most probably notice the following presets: ultrafast, superfast, veryfast, faster, fast, medium, slow, slower, veryslow. Ideally you'd always use veryslow when rendering, but you can't always wait for it to finish, so you go for faster ones. The ffmpeg wiki summarizes the difference between these presets:
veryslow helps about 3% compared to the slowerpreset, slower helps about 5% compared to the slow preset, andTo be able to use for example slow with ges-launch-1.0, you need to create a GStreamer preset. Add the following section to ~/.local/share/gstreamer-1.0/presets/GstX264Enc.prs
slow helps about 5-10% compared to the medium preset.
You need to specify a bitrate in kbits per second because the default value of 2048 kbits per second might not be what you want. I picked 12288 because it's what YT recommends for 1920x1080p50. See the documentation for the x264enc plugin to see all the options you can specify in the preset.
You might also be interested in specifying a tune:
or a psy-tune:
Other tune values are: film, animation, grain, psnr, ssim. For the list of available tune values see the plugin source code.
To use the new preset, run:
$ ges-launch-1.0 -l pitivi-project.xges -o out.mp4 -f "video/quicktime,variant=iso:video/x-raw,width=1920,height=1080->video/x-h264+slow12mbps:audio/mpeg,mpegversion=1,layer=3"
The -f parameter accepts a serialized encoding profile.