I used the version of Pitivi available in Ubuntu 18.04 yesterday and for the life of me I could not get it to render anything but potato quality video (500 MB file for 60 minutes at 1080p).

The deb from Ubuntu 18.10 had unmet dependencies, which is not a surprise given how interwoven it is with gstreamer.

Finally, I installed the most recent version via flatpak and it rendered fine.