Hello.
I have to edit a video, pretty simple things like removing parts, putting something else in that place, synchronizing the whole with a separate sound track.
YET, Kdenlive can't render anything: when choosing the lossless format FFv1+flac, the resulting .mkv doesn't last more than 5 sec, whereas the first chunk of video, before the first gap in the timeline, lasts more than 3 minutes. Though I am no expert in video editing, I don't think I can be called a dimwit in computer stuff. And that's clearly not a normal behavior... And the rendering process lasts about 10 sec, which is pretty short. It does the same with any format, or it doesn't even start rendering, or it stays an indefinite amount of time on " 0m remaining", which makes "0m" lasts quite a bit of time.
I can still render the audio though, and there it takes its time, 6 to 10 minutes, which seems more appropriate.

I have now the last AppImage version from the Kdenlive website.

I'm using DIVX (or.. better H.264) without issues on rendering... have you tried to reduce (maybe) resolution of project to 720? (i'm using this resolution now.. but i think that 1080 works also very well)

the rendering issues happens also when you do not do modification of file? (just placing the original video on track 1... and render?)

What I did so far was cuting the original files here and there and filling them with space.
Actually I don't even know how to change the resolution like this... I checked the output format, but saw nowhere somewhere where I could choose the resolution as easily as in youtube. I doubt it would change much things though, as I tried many different formats.
I tried letting in work during the night, and that work which was written to have to last 43 minutes, wasn't even started this morning. But the computer was hot.
I'll try it again this night with the DIVX format.

you can also try to rendering just a "section" of the video.. just to see if the renderer works... and to see quality.
there is "selection" in the timeline (seconds/minutes line) .. that you can move or expand using mouse..
on the render window there is the flag to render just this small section.