I thought that with videoalpha the video speed would be interpolated for you (so you wouldn't need to specify multiple speeds). That is not correct?

You could use a xblock field of type List. However, that would not allow you to specify which speed corresponds to each entry.

There is an xblock field of type Dict. That would allow you to specify the key (speed) and the video. 

We currently don't have metadata editors for either of these.