I am surprised at just how important variable speed has become to me. I have found myself needing this effect so many times over the last few months (for all sorts of different reasons) but unfortunately AVS is still unable to do this.
I was looking at my old video editing software (ULead Media Studio v8) and although that program is way too difficult and complicated to use, it does have a very good variable speed function. You open a clip and then click on the timeline to create speed points. For each point you specify the speed that you want the video to be playing at when it reaches that point. You can enter any value whatsoever between 10% and 1000% (accurate to a single digit) so you are not restricted to large 25% jumps like you are with AVS. The video editor then interpolates between the values entered on the speed points and very smoothly transitions between each speed value.
So, for example, you could add a speed point at the beginning of a clip, one in the middle and one at the end, and if you then assigned values to these speed points of 107, 13 and 286 you would get slightly faster than normal playback to begin with (7% faster than default 100%) and then it would gradually slow down to 13% of default speed and then gradually speed up to 286% (i.e. approaching 3 times normal speed).
I have used Ulead Media Studio for syncing out of time action to a regular beat from a newly added soundtrack (because you can make various adjustments to speed up and slow down as needed to keep things in time with the music). The speed changes were so smooth that you couldn't tell playback speed was altering, it just looked like the action was done in time with the music.
Something like this would be really great for AVS Video editor!