...
I'll tell you my story.
Have: 27-inch Late 2012 iMac, 3.4GHz i7, 32GB RAM, NVIDIA GTX 680MX 2GB
Got: Base model iMac 5K Retina, 3.5GHz i5, 8GB RAM, AMD M290X 2GB
...After some basic use over the next hour, I noticed the 5k did not seem as smooth, with web browsing, popping up a finder, using mission control. Just slight millisecond pauses here and there, granted I was trying to torture test it, but I was expecting it to be more like my 2012 but only faster.
.... Or maybe I should have added more RAM like I planned. Or I should have waited the month for the top model to ship to me.
I just noticed your base 5K vs beefed up 2012 reference.
It's pretty clear. That's entirely what happened. A general rule of thumb is RAM is the single biggest thing to speed up any system. Period. A 4GB system vs 8GB system is like night and day, because just about everything involves a re-read from disk.
If you compare the responsiveness of any computer with 8GB with any other computer with 32 GB, the system with 32GB is going to crush the 8GB system, because almost everything you will do will be buffered to physical RAM rather than virtual memory swap files or re-loading from disk.
Even with a 10-15% speed increase from a 2012 3.4 Ghz i7 to a Haswell 2014 3.5 Ghz i7, the gap between 8GB and 32 will still give the advantage to the older system.
I was tempted to get a stock model from the Apple Store because I could get it the same day, and I knew I could add RAM in a couple of days from OWC or Crucial, but since they don't stock models with the upgraded CPU, GPU or SSD drives, I simply had to wait for a custom build. I ordered the 4.0 Ghz CPU, 512 SSD and the upgraded GPU... It was clearly worth it.
This iMac 5K is smoking fast. The upgraded GPU has more than enough horsepower for the 5K screen. I think it's rated at about 3.5 Teraflops, which is on par with the mid-level Mac Pro's D500 GPU performance.
I did one of my own benchmarks and this iMac was more than twice as fast as the entry level Mac Pro in Final Cut Pro X at background rendering, and several times faster at H.264 compression. I admit the compression test was unfair, because Xeon chips don't have Quick Sync, which is hardware based H.264 compression.
That said, a 2012 or 2013 iMac with an i7 is still a great system. If you tried at least more RAM, you would've had a fair test, but it's hard to see drastic improvements in speed in a move to a model only 2 years newer.
I had a similar issue. My 2011 i7 MacBook Pro 17" cost $3000 and had all the upgrades, and I even added more afterwards by jumping to 16GB and adding a third party SSD. It was hard to justify spending another $3500 since the old one was pretty fast.
The only thing that made me pull the trigger was that 2011 MacBook Pros are having GPU failures, and when mine started showing signs, I made the move.