USB 2.0 will max out around 30 MB/s, or a little under half of what a spinning 2.5" drive is capable of.
I don't know about iMovie in particular but I would recommend finding somebody (friend, relative) with a 2.5" drive that you can try out, copy some files onto it, and try working with those. Should be fairly easy to do.
I've had 6-7 different external drives, both USB 2 and 3, and I've never seen huge differences between performance.
USB 2 drives all max out at around 30 MB/s because that's the limit of the standard.
USB 3 drives should all run at the drive's speed. USB 3 is capable of 400+ MB/s, so even if you have a s***** controller or a s***** cable you're going to be able to get ~100 MB/s which is basically the maximum that a spinning 2.5" disk will give you.
I wonder if something is wrong with how you're testing performance?