I was finally able to install Kdenlive Version 19.08.2 on Lubuntu Bionic Beaver (apparently it used a "snap" package to install in case that matters).
My computer is older (PentiumD dual core) but it works fine with KdenLive 17. Also, I am only using Standard Definition MiniDV camera files (NTSC mpegs) so it should not be a load for a computer of this age.

In Kdenlive 19.08.2 something strange occurs that does not occur in the older version 17. When I view a MiniDV clip via the Clip Monitor window, the video plays smoothly and at the proper frame rate. However, when I put the same clip on the timeline and play it (view it through the Project Monitor window) the video plays back very choppily - i.e. at 2 frames per second. I am not using any effect, fades, etc. This is just during straight playback of the unaltered clip.

Does anyone know why this occurs? Is there anything I can do to make playback in the Project Monitor as smooth as it is in the Clip Monitor?

Thanks

i have a choppy playback (from timeline) on a Xeon 12 thread. (1080p) CPU usage is 12%, i'm on window and seems that timeline preview is using only 1 core (and no GPU)

I can confirm that my CPU usage goes up to 100% when trying to play from the timeline, whereas it's only 50% when playing from the Clip Monitor.