It is my impression that fast is not the issue. Not really. Not in Lightroom. Not like with video. There are experts who prefer to edit everything on an external drive so that they can use their desktop and then their laptop and always have their photos with them.
There is rarely a noticeable speed difference in editing due to speed of the drive where the photo is stored. You can put the photos on the external drive and leave them there. The extremely minor speed difference you would get from putting the photos on your SSD would be dwarfed by the time it will take you to move the photos from one disk to the other. Editing speed is almost entirely determined by CPU speed, if you want faster edits, get a faster CPU.
Also, see this study by Ian Lyons: http://www.computer-darkroom.com/blog/will-an-ssd-improve-adobe-lightroom-performance/
Now, since you are concerned about speed, tell us the real problem. What isn't fast enough? Exactly what actions in Lightroom are slow? How big are these photos you are editing in megapixels (not megabytes)? How large is your monitor in pixels?
I agree with dj_paige with the addition that we now get a benefit from our GPU as well.