So exactly what would be an ideal minimum processor for the usage of Logic and additional plug-ins?
The minimum I would suggest in the 21.5" model for any creative production work is:
a quad-core processor,
8GB of RAM, and at least a
1TB Fusion drive. 256GB SSD would be better (costing more money), but then you would probably have to output to an external drive (costing even more money).
I use the above system (w/1TB Fusion), and it handles my hobbyist activities in Adobe Premiere, Photoshop, and Illustrator well. There is no lag opening the handful of applications I use. Outputting video from Premiere is a tiny bit slow, but I knew that going in. I can wait. It usually ends up being 1.5 to 2 minutes rendering for every minute of footage. Not bad. I think the trade-off the 5400RPM drive is decreased heat, thus a more stable, less throttled, and longer-lasting system.
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Regarding the 5400RPM drive everyone has been freaking out about, and some perspective:
I have done some testing using the Blackmagic speed test on one of the 2010 iMacs we have at work...comparing its speed to my current 2015 model. I work in the print industry, prepress and design.
2010 iMac a quad-core processor, 7200RPM hard drive.
2015 iMac has a quad-core processor, 5400RPM drive.
I am getting between 250-325 MB/s write speeds the first couple of tests on my 2015 iMac (that's the SSD). Third or fourth test in write speeds drop to between 150-180MB/s. That tells me it's hitting the spinner.
Tests on the 2010 model show consistent speeds of 50-80MB/s (
maybe up to 100 MB/s sometimes?).
So...my limited testing shows, that while 5400RPM on paper
looks inferior to what Apple has used in the past for hard drives, the slower 1TB drives are actually twice as fast the "better" drives they were using five years ago. I am hoping this was a design choice to reduce heat and creating a more stable system (second to the profit margins

). The 5400RPM drives are not particularly fast, when compared in this new environment where SSDs are so prevalent, but they are also not really SLOW using the standards we operated under just a few years ago. Something to keep in mind.