See the MPG review of the 2017 iMac 5K.
In a classic case of not assuming too much, MPG tested the 2017 iMac 5K SSD speed. Apple likes to quote peak speeds, but all is not well for the vast majority of applications, which perform the bulk of their disk I/O in 1MB or smaller chunks—even Photoshop CC 2017 (so far at least),
Read more: iMac 5K (June 2017): Flash Drive (SSD)
The late 2015 iMac 5K SSD outperforms the 2017 iMac 5K for the critically important transfer sizes up to 1MB, particularly for writes, where the 2017 model lags badly. Many if not most programs do not read or write data in chunks larger than 1MB, so these results mean that the 2017 iMac 5K will be significantly slower than the 2015 iMac 5K whenever small files or I/O sizes are involved.
Reading a mix of 217000 files of varying sizes (html files and JPEG) shows that the performance is slightly in favor of the 2015 iMac 5K with a mix of text files and large JPEGs, but about 40% faster on the 2015 iMac 5K with small files (html files of a few kilobytes). This argues that things like file copying, or searching/grepping, git, etc will be faster on the 2015 model. Thus the 2017 iMac 5K is substantially slower for everyday tasks that involve intensive I/O with smaller reads, in spite of the 40% greater speed for large reads.