I have not been able to figure out how to apply transitions with a clip that has the speed effect applied to it. I'm not certain whether this is a bug or a conceptual problem on my part. However, using the speed effect on a clip appears to cause transitions to be ignored (a sudden cut occurs both when previewing and when viewing the "rendered" video).
To repeat this (tested in Kdenlive 0.7.x and 0.8.2):
1) Load a video clip (it does not appear to matter what the source is)
2) Cut it into two parts so you have some video to use
3) Apply the "speed" effect to the second clip (it doesn't matter whether you apply it to the first or not)
4) Overlap the 2nd clip over the first (doesn't appear to matter which track is on top)
5) Apply a transition such as "dissolve"
My expectation is that a dissolve from the first clip to the second will occur, along with the speed effect. What happens in fact, though, is that the dissolve is ignored and does not preview or render (exactly as if no dissolve were added).
This doesn't appear to happen with other effects (e.g. "old film" or "flippo"), which makes me think this is an actual unreported bug (at least, I haven't found a bug report that matches it). But as I'm pretty new with Kdenlive, I thought I should ask and see if maybe I'm just doing it wrong somehow.
Another thought occurs to me -- if Kdenlive had a "bake" or "pre-render" option, I could route around a bug like this by applying a transition to the "baked" speed effect (of course, it is possible to do this manually, but is a PITA).