In most normal situations the question is irrelevant. You're not waiting for the CPU; you're waiting for the data to be read from and written to the scratch disk. The tests above are flawed; they were conducted with a single SATA 6GB/s drive, which is slow enough to mask the real result. In addition only 8GB RAM, so not enough to help.
If you have lags and performance issues in Photoshop, that's not the reason.
Yes, some filters are CPU-intensive, but more and more of that is moved to the GPU these days.
(edit: ...oh, and why is it called SATA 6GB/s, when the real throughput is at best 450 megabytes/sec?)
I just noticed that the test above (from 2015) not using just 8 GB RAM, it uses 8x 8GBs.
OK, but that's still only 64GB, not nearly enough to contain a real-life scratch file. And in any case it's always written to scratch even if it could be contained in RAM.
So the point still stands. Photoshop is not CPU-limited in most normal scenarios. It is multi-threaded, it does use 8 or 16 cores in parallell where possible (think nine pregnant women) - but that's just not what you're waiting for.
@D.Fosse "why is it called SATA 6GB/s, when the real throughput is at best 450 megabytes/sec?"
SATAIII bandwidth is 6 G bits/second which is 750MBytes/sec. However the transfer process also uses 10bit words in the transfer process (known as 8b/10b encoding). So each 8bit word uses 10 bits of bandwidth, reducing the rate further to a max theoretical 600MByte/sec of data transferred. That of course is theoretical across the SATA interface and the SSD drive then has a maximum read and write speed which is short of that maximum SATA transfer speed.
In addition, bit transfer is measured as 1000 bits = 1 kbit , 1000 kbits = 1 Mbit, 1000 Mbits = 1Gbit. However file storage is measured as 1 byte = 8 bits, 1024 bytes = 1Kbyte, 1024 Kbyte = 1MByte, 1024 MBytes = 1 Gbyte storage.
So even if a drive could hit the maximum SATA transfer rate of 600MBytes data/sec, that will only equate to a file storage size of 572MBytes transferred.
It depends what functions you are using. for example 3D ray tracing in Photoshop* uses every available logical core but some 2D filters do not. As Dag said - scratch disk and RAM will have the biggest impact rather than extra cores.
*If you are going to be doing a lot of 3D ray tracing , I would advise using a 3D application that uses the GPU for that function where you can utilise thousands of cores and cut rendering times accordingly.