You just described my set up.

I run all my software off an SSD card but I store my photos on a 1.5 TB HDD. It works well. I do think there are the occasional bottlenecks. Basically, sometimes the HDD "goes to sleep" and can take a bit to wake up. But for general processing, there isn't much of a delay as that does not involve the HDD very often. Instead, at least my understanding, the images while working on them in LR are stored in the RAM. So, did you get much RAM? I have 8 GB DDR3 1600 and that seems to work well. And...of course, the new LR utilizes your GPU as well as your CPU for processing power.

Do be careful when importing. Sometimes LR wants to import the new photos to the SSD. Not a big deal to move them over to the HDD, just more work.