Is it generally recommended to have the catalogue and previews on the fastest drive?
I can't quite work out whether having the previews stored on an SSD-only iMac will increase the speed of load time and by how much. (I read somewhere it might be between 2-5% of the original Raw file size but not certain.)
What load time are you referring to? Load the image into the Develop Module? Load the image in the Library Module? Zoom to 1:1?
I am also not certain how big the size of the preview file will end up in 1:1 or smart preview mode (currently, I think it's set to low or medium and the file is around 60GB in size).
This ought to be easy to figure out. Figure out the size of your previews now, then (for example) import 100 images specifying 1:1 previews, see how much space the previews take up, do the math.
Any advice for (near) optimum performance? thanks
Performance? Performance of what? Usually when people ask about performance, they are talking about slowness in the Develop Module (in which case SSD won't make the slightest bit of difference). Is that your concern? Or something else?
My biggest problem with LR was always the long waiting times between scrolling through images in Library mode (they loaded too slowly). Does having all Previews in the SSD play a part in this? The other was the Develop module (applying presets to an image for example would take a long time, but I worked on an old MacBook Pro from 2010, 2.4 GHz Intel Core 2 Duo, 8GB of RAM. Presumably the new iMac's 3.3GHz quad-core Intel Core i5 processor and 24 or 32GB of RAM should improve things). I can't work out if I am making a mistake going for fusion over pure SSD. I store media on external HDDs anyway.
If you generate 1:1 previews and keep them on an SSD, that ought to give you the fastest scrolling in Library Module.
I think you are making a mistake getting an i5 and lots of memory, I would get the fastest i7 CPU you can afford, 4 or 6 cores, and 16GB memory. Speed in the develop module is primarily driven by the CPU (and GPU if it is enabled). As long as you have enough memory (16GB is enough) additional memory will not noticeably speed up develop module actions.
I agree with dj_aige. You would be much wiser to purchase a fast i7 processor with 16 GB RAM. I have my catalog and preview images on a SSD drive with my images on other internal hard drives. If you have the catalog and previews on the SSD drive that will give you the best performance. Having your images on an SSD drive probably isn't going to make much difference.