The zoom speed is directly proportional to the amount of zoom and time. For instance if you want to do a 20% zoom and you set the duration to go from zero to 20% at 10 seconds that’s a slow zoom. But if you do that same zoom from zero 20% in half a second it becomes a fast zoom.
Here’s the mlt for the two zooms. You need to substitute your own images. Look at how the filters are applied.