In Settings > Configure Kdenlive > Environment there is a setting for "MLT Processing threads" which is at 1 by default and allows (on my machine) a max value of 3. Now clearly this is about multithreading MLT, but I'm just wondering which parts of Kdenlive this setting would affect?

Playback in the monitor?
Rendering?
Both?

It also says >1 is experimental. Does anyone have any idea of *how* experimental it is? Do you have any experience with trying this?

I used the sunab 0.92 release ppa with the 0.8 mlt.
I have the mlt threads on 3. I definitely notice the difference when rendering going from 1 thread to 2 to 3. I have a quad-core i7 which supports 8 threads.
Going higher then 3 doesn't seem to make a big difference so I prefer to keep those threads for other processes.
And the fact that it's experimental: I do quite a lot of (partial) renederings to see the result. I never experienced a quirk in the rendered movie clips (so far).

I don't notice a difference in playback.

I just installed mlt 0.82 by the way. I don't have experience with that version yet.

A long time ago, I tried that at a higher rate but it made the timeline stutter like mad, so I steered clear of it all together. Might be time to experiment again. Cheers.

Using multiple MLT threads is still not flawless. I thought it worked well and it it does work well with "normal" rendering and static titles.
However, I did make this time long horizontal scrolling titles and long vertical scrolling titles. They stutter immensely resulting in a horrible video. Using only one thread for the MLT process resulted in a fluent scroll (but 30% longer rendering times: so be it).