HI
I am assuming from what you have written that you are changing the clip speed prior to adding it to the timeline. (It is possible to do this by right clicking the clip in the media list and selecting "Change clip speed".) In version 3.12 I find if you do this, the "Property" duration of the clip itself remains as it was, but if you drag it to the timeline and examine the duration in Storyboard mode you will see that it has changed. However I find that the frame of the clip now shows a black rectangle instead of the usual thumbnail. This must be a BUG!
e.g...
I placed a clip on the media list with a "Property" duration of 6 seconds. i.e. Right click and select "Properties"
I first dropped this onto the timeline in Storyboard mode to check all was well......it was. The time shown was 6 seconds and the clip played normally.
I then deleted this from the timeline leaving it empty.....(Back to square one)
Now I left clicked the clip in the media list and selected to "Change the clip Speed" and changed this to 200 so nominally the clip would now play for 3 seconds.
The "Properties" correctly showed it to still be 6 seconds duration.
I then dragged the clip to the timeline and found that the duration there showed 3 seconds which is what I expected but there was no thumbnail and the frame was black. 
Switching back to timeline mode at this point, both preview panes correctly showed the clip and sequence to be 3 seconds and also showed the preview image. However the thumbnail line on the timeline track line showed NO thumbnails, just the "Landscape icon". This must be another BUG!
I then played the sequence and found that the clip preview then went black.
In fact I deleted everything and started from scratch again with the same 6 second MPEG4 clip. This time I dragged and dropped the clip straight to the timeline intending to change the clip speed from there.Unfortunately although the clip previewd OK in both windows there were, once again, NO clip thumbnails on the timeline. 
I then right clicked the timeline and changed the clip speed to 200% as before. The time bar changed to 3 seconds as expected but there was no green progress bar and the preview was extremely choppy, probably because it was still rendering the speed change. Even after some minutes there were STILL no timeline thumbnails.
Switching to storyboard mode revealed that the clip was now 3 seconds long but still with a Black Frame!
Slowing down the clip to 50% speed seemed to work slightly better in that, at least there was a thumbnail to be seen in the storyboard frame. But there still no thumbnails to be seen on the timeline track. Playback was still VERY choppy.
This MAY be because I was using an HD MPEG4 clip on an older computer, but all I can say is that I have no problems doing this particular manipulation with version 2.41.
So IMHO there are a lot of bugs still to iron out in the 3.xx versions. Some them have been mentioned before in the forum, like the black frame that appears when clips are cut and joined and the inability to show some images in the rendered output. It is also too slow to make any editing interesting and some areas simply don't work correctly.
I personally find that VP 2.41 is the most satisfactory version to work with.
Nat