So, I just lost 1-2 hours of work, because I dared to move some clips with the speed effect applied to a different position on the timeline.
Basically what I did: take a clip, cut a piece out, apply speed effect
Watch it: displays what I cut out, but in fast forward, so, all fine
Now move it to a different location on the timeline.
Watch it: It displays a completely different part of the clip I cut, making it pretty much useless
Reloading the project now will make the clips appear without the speed effect at all!
So, I have to cut, and apply the effect again, and I need to know where that is going to be in the end, because as soon as I move it, I must trash it.

Yes, there is definitely a bug in the Speed effect when moving clips, and it's been there for quite a while. I came here looking for a possible permanent solution but I can see that there is none. I can, at least, offer a workaround that I found to prevent losing hours of work, as I also have done.

Anytime you do just about anything with a clip that has already had the speed effect applied, drag your timeline cursor through it to be sure it did not get corrupted. Most likely it will. The workaround I found is to delete the clip then immediately undo the delete. Drag your timeline cursor through the clip again. If yours is like mine, the clip will be back to normal.

A permanent fix to this bug would be nice, though.

I get the same problemS:
- speed effect not saving. Randomly , for some clips it works, maybe those clips that's i didn't move afterwards
- after moving the clips, the source changed,... making me loose quite some time working,

i am using:
Version 16.12.1
KDE Frameworks 5.28.0
Qt 5.6.1 (built against 5.6.1)
kubuntu 16.04.1

bug already reported for a while, on 16.08 :
https://bugs.kde.org/show_bug.cgi?id=372819
and a video showing the bug:
https://youtu.be/Ez_TTw1g4sU

We are doing a timeline refactoring to solve this issue as well as bring advanced editing tools. Stay tuned for more info. But for now I would avoid the speed effect like the plague if I were you.

thank you for warning....

... and i can imagine that nothing will come before a month
I'm behind you 100% !!