i always search for simple solutions... so was ment my sharing here.
and its simple and cheap to invest 170 -199$ in an older mbp, stick the card in the express slot, format it and double the speed of that machine at least with photoshop but also lightroom.
no,- gwith, the card doesnt stick out. just 1 millimeter and thats that it can be pressed in a bit it to eject it.
btw. : i use a filemate solidgo expresscard with 48gb, price between 170-200$, 115MB/s reading speed, 65Mb/s writing.
part of the increased speed seems to be that SSD is with many small files ( as the scratch file or camera raw file is ) much faster than the pure numbers tell, cause there is no head to be moved.
of course you can tune the box faster investing several thousand bucks and much energy, but mine works with my over 2 year old laptop and it does it very good. thats the point of what i wrote ...