Is there any way to achieve smooth slow-motion at 50% speed? I edit 1080/25p footage and it's pretty choppy. A quick search with Google told me that ffmpeg does support interpolation for a smooth slow-motion (or is this only for interpolated footage?).

Maybe this would help:

http://jcornet.free.fr/linux/yuvmotionfps.html

I haven't yet tried it myself....

FFmpeg does interpolation for scaling and deinterlace but not temporally for frame rate reduction. There was an attempt made in MLT as used by Kdenlive, but it was not the primary goal of the author's work, and does not give good results at all. Basically, it's not available in Kdenlive.