There is an extremely small, trivial, performance loss by putting photos on a slower disk, so small that you will never notice it. This study by Ian Lyons confirms this: http://www.computer-darkroom.com/blog/will-an-ssd-improve-adobe-lightroom-performance/
In theory, there will be a performance difference between the SSD and spinning hard drives. In practice, whether or not you see it in the real world is open for debate.
Your performance gains in Lightroom is going to come from having high-end CPUs and lots of RAM.
The GPU optimization may or may not help you.
My 64-bit Win10 box (8 cores, 32 GB) has the catalog and preview files on a spinning drive, separate from the spinning drives (internal and external) containing the original images. Size of the preview files is set to "auto" to match my monitor; no smart previews; GPU optimization turned off. I don't notice any lags in the Library or Development modules. (Importing and exporting take however long they take.)
Just my thoughts, Lightroom by default reads and writes constantly to the Catalog file and the preview files. So if you have these files on the SSD then this is the best place for them provided there is lots of free disk space on the drive. You will tend to see a diminished performance if the free space drops below 25% of the Drive capacity. e.g Drive capacity 500GB minimum free space 125GB. (To wit a SSD 500GB drive with 50GB,10% of free space may not perform better than a 4TB HHD drive with 2TB, 50% of free space)
With respect to the "slow HDD" this will depend first if the disk is internally connected or an external drive i.e USB 2 or 3?. If you do not have "Automatically write XMP to the files" then Lightroom is only reading from the files so there would be limited performance issues. If the HDD is USB 2 externally connected and you are auto writing to xmp then this may be the weakest link to performance.