kdenlive master branch commit v17.08.1-35-gf2b70a17c I have always had various problems with speed effect ranging from silent time line corruption, unmovable clips, broken end and out points to crashes during playback. Thus I was a bit afraid of using it on my latest project. These were an indication that a serious rewrite and tests was needed for the effect at that time (past 7 years or so). Luckily I had not seen time line corruptions or crashes now, but the playback of clips with speed effect on time line stops, if the clip is moved in a group. Have not tested if this happens in rendering to but at least on the instant preview this happens every time on my project. Tested various 17.08.1 git commits and they all behave like this. Workaround is to enable and disable the effect provided that there is enough space on timeline for that. But no crashes or timeline corruptions, woohoo! :)
Sigh, sorry. Spoke too soon. Now I have ghost clips, unpreviewable still images, broken in and out points etc on timeline. Disabling and enabling speed effect from the clip causes it reduce in length and in and out points go way off the originals. All this seems be caused by moving groups of clips which have speed effect applied. Tried to use undo changes to this but it seemed to make things worse. Need to manually redo large sections of timeline now. Similar things happened already several years ago sadly. I think speed effect needs a big fat warning sign.
Fixed in 19.04