I have a 5k iMac running lightroom (I'm aware people have performance complaints). I have a fusion drive but my images are stored on a Lacie Rugged thunderbolt drive. The only performance issue I really have is the "loading" screen message in library and develop modes. I was considering upgrading my internal drive to an SSD but seeing as my images are on an external drive would it help much? I could test on my MacBook pro which has an internal SSD, but seeing its screen is half the resolution Im not sure it would be a solid comparison.
Many people report performance issues with 5K Macs, but putting photos on an SSD will not help with loading delays. Putting the catalog file on an SSD might help, but the real problem is the CPU isn't fast enough to drive a 5K screen through Lightroom, the calculations required take long enough to cause this delay.