IMO LR6 and Photoshop CC can have sluggish aspects on a top-spec 2015 iMac 27. I never noticed this on my 2013 iMac 27. There is obviously something suboptimal with Adobe's code because on the 2015 iMac certain effects in LR get faster if you turn the GPU off. In PS, filter preview for smart sharpen and other effects can take several seconds to render. IOW when you click the preview checkbox on/off within a filter, it can take several seconds, making quick before/after comparisons difficult.
It seems very likely this is yet more sluggish Adobe code -- I don't think other photo editing programs are this slow. It is also in line with the performance differences between Premiere Pro CC and FCPX, which is much faster at many tasks. It is also in line with Adobe's need to issue a public apology for the poor state of LR 6.2.
It is not unusable but (depending on your workflow) can feel sluggish and laggy. If you work slowly and methodically you might not notice it. If you're a professional event photographer, work quickly and blitz through photos quickly, it can be a little frustrating. However you adapt to the peculiarities of the hardware/software platform.
None of these are I/O related, so SSD vs Fusion Drive makes no difference. Most time-consuming operations in LR such as import and 1:1 preview generation are CPU and/or GPU bound not I/O bound.
In general I'd suggest the top-spec iMac 27, except for memory and disk. At your discretion you can get minimum RAM then update that yourself with aftermarket RAM and save some money. Regarding disk, there is often limited real-world performance difference between SSD and Fusion Drive. If you only get 256GB SSD you will probably be storing much of your data externally, which is OK but you'll need a pretty fast disk else it would have been faster to just use Fusion Drive.
