Yes you can create a Fusion drive between the internal and an external disk. It's a destructive process, so be sure to backup your system first.
I don't find the Fusion drive to be noticeably slow, but I only have about 150GB on my machine as all media/library/archive data resides on network storage. So most of the local data resides right on the SSD portion of the Fusion drive.
So why the disappointment in the fusion drive?
Destructive process?