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iMac 5K: Bridges the Gap to the Mac Pro
As noted in iMac 5K (Late 2015) with 64GB: the Key to Professional Usage, the late 2015 iMac 5K changes the game by accepting up to 64GB memory, which makes it viable for professional photographers like diglloyd, videographers, etc*.
Sweetening the deal considerably, the iMac 5K SSD runs rings around the 2013 Mac Pro SSD (at least an early 2014 'build'), and the iMac 5K CPU (only 4 cores) stomps the fastest possible Mac Pro on Lloyd’s most important and time consuming real world task.
Frosting on the cake is Thunderbolt networking, which means that the iMac 5K can be a high performance workstation able to read huge Photoshop files nearly as quickly as with a local fast SSD (Photoshop has some internal limits, so a gigabyte/sec is plenty fast).
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iMac 5K general limitations
The iMac 5K has some drawbacks. Some of these are subjective, some speculative, and some are hardware related.
- Fan noise can be louder than a Mac Pro, because the iMac sits directly in front of the user whereas the Mac Pro can go under a desk. Even so, your author has the 2013 Mac Pro on the desk (albeit further back than the display, which helps slightly, another advantage), and its fan noise is rarely noticed. Not so with the iMac which makes itself heard more often. The iMac fan tends to kick in sooner under similar processing loads, perhaps because 4 CPU cores is all of them (full load), yet the 8-core Mac Pro is only half utilized when 4 cores are in use. And the Mac Pro simply has a more robust vertical cooling system and fan designed for very heavy loads.
- The all-in-one iMac design is packed into a tight case and that includes the display. If the display fails, the computer cannot be used; with a Mac Pro the display can be replaced.
- Longevity: is the iMac 5K as robust after 2 or 3 years of moderate to heavy workloads in which the GPU and CPU become hot? What about cooling, and the accumulation of dust internally (hard to clean an iMac; the Mac Pro shell slides off trivially).
iMac 5K hardware limitations
The Mac Pro can go to 128GB for really huge jobs.
The iMac 5K has only two Thunderbolt ports on a single Thunderbolt 2 bus. This is a headache for connecting multiple Thunderbolt peripherals, since they have to be daisy-chained (connected one after the other in series), and thus all devices must be powered on for anything further down the chain to operate. The 6 Thunderbolt ports of the Mac Pro (on 3 Thunderbolt busses) give a lot more flexibility for users with multiple TB peripherals.
With the iMac 5K, a display should be placed at the end of the chain since it dead-ends the Thunderbolt chain (as this was written there were no Thunderbolt 2 daisy-chainable displays). Alternately, a display can be plugged directly into one of the two iMac Thunderbolt ports, but then only a single port remains for (daisy chained) devices.
There are also Thunderbolt 2 bandwidth limits for high speed device-to-device copies, backups, etc. For example, Thunderbolt networking will utilize much of the bandwidth and this would degrade the speed of any high-speed device on the Thunderbolt chain (either port).
Most of these hardware limitations are of little concern for most users and usage scenarios. Also, they can be mitigated by utilizing the iMac 5K being as a “compute station” with its own high-speed internal SSD (flash drive), along with Thunderbolt networking to high capacity storage via file sharing on the 2013 Mac Pro and/or to other Macs with their own high speed flash storage. Think 8TB OWC Viper for a small workgroup: gigabyte per second speeds over Thunderbolt networking to a shared 8TB workgroup volume.