I'm quite new to Kdenlive (v 15.12.3), though I used to to a lot of NLE work many years ago.

My immediate need is quite simple, just to trim the ends of some videos, and perhaps add fade-in/fade-out effects, no change of format. I can easily do all this on the timeline, but the rendering speed is terrible. Apart from the first and last second or so, the rest of the video is unchanged, so rendering should just be a copy process. The NLE I used to use would do this automatically, only re-encoding where necessary.

However, it seems that Kdenlive always decodes and encodes the entire video stream, which is completely unnecessary. I've found that for just trimming the ends, without adding fades, it's quicker to use Kdenlive to find the cut points, then run ffmpeg directly using "-codec copy" (quicker by a factor of 100!), but that's all rather tedious.

It appears that Kdenlive actually runs melt underneath, and experimenting with running melt on the command line, it won't accept "copy" for a codec name, unlike ffmpeg. Is there an option I haven't found for forcing it to do that? The documentation on melt is sparse to say the least!

Copy rendering is pretty fundamental for an NLE, I find it hard to believe that an NLE as powerful as Kdenlive can't do it (it also avoids quality loss, which is inevitable with re-encoding).

No. Kdenlive does not support this feature. Avidemux could be you friend for that kind of editing.