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.
I just stumbled on this forum post looking for best ways to setup Lightroom. The article listed as the best answer is really outdated now. The SSDs m.2 are nor over 3,000 MB/s read and 2,000 MB/s write. I'm sure those numbers would change a lot now. Also, Lightroom's version could probably changed those numbers too.
I'm not saying the article is bad or anything like that... just that those numbers might not be the best ones anymore.
The article may be outdated, but the underlying architecture has not changed. If old disks can send the photo to Lightroom in (let's say) 1 second, and the new faster SSDs can send the photo to Lightroom is 0.1 second, is that really an improvement that will make a difference to you?
But furthermore, a lot of people think develop module speed depends on disk speed. This is false.The underlyng architecture of Lightroom that I mentioned means that Develop Module speed depends almost entirely on the CPU speed (and if enabled the GPU speed). The disk where the photos are stored is not involved in editing, and so a faster disk will make no difference to editing.
So, echantigny , essentially I disagree with your entire statement.
You are totally right about CPU and GPU, I'm not arguing that. Im just arguing that those number would definitely change nowadays if you use an SSD m.2 to import your photos... But most people save the photos on a normal hd so that doesn't even change much.
As I said, if it takes 1 second on an old spinning disk to access the photo, and 0.1 seconds on the new fast SSDs, is that really an important improvement to you? Importing accesses the photo (let's say 1 second on an old disk), then it renders the photo which takes CPU (and not GPU) time, probably a few seconds or more depending on how big the photo is, and than a short amount of time (probably 1 second or less) to write th e previews. Again, importing is a process that depends primarily on CPU speed, and yes, you might be able to save fractions of a second with faster SSD drives, but if fractions of a second are that important to you, then go for it. The SSD does not affect the huge majority of the time it takes to import the photo.