New 5K iMac GPU configurations at least double best performance of MacBook Pro
As with the initial Radeon Pro release with the 2016 MacBook Pro, Apple hasn't provided that much detail on the same family of GPUs used in the new iMacs -- but AMD has shed some light on the new processors, showing a radical improvement over the predecessors.

According to AMD, The Radeon Pro graphics found in the MacBook Pro and iMac feature the latest "Polaris" architecture with 4th generation Graphics Core Next. As with the GPUs in the series in the MacBook Pro, the new iMac GPUs feature AMD's asynchronous compute technology, updated shader engines, enhanced memory compression and new geometry capabilities.
The speeds in the new Pro 570, 575, and 580 beat out the MacBook Pro's set. The Pro 570 nearly doubles performance of the Pro 460 and 560 in the highest configuration of the MacBook Pro possible. The Pro 580 is 2.9 times faster in single precision than the Pro 560.

In comparison, the Iris Graphics 540 found on the 13-inch late 2016 MacBook Pro has a peak performance of 806 gigaflops, with the 550 in the higher end configuration coming in at 845 gigaflops. The 2013 Mac Pro desktop GPU D300 FirePro is capable of 2 teraflops per GPU, and the computer holds two. The D500 FirePro can provide 2.2 teraflops per GPU.
The Nvidia 1080ti with a native PCI-E connection for around $700 delivers around 11.3 tflop, with the $500 1080 coming in at 9 tflop. The $1200 Titan Xp hits around 12 tflop.
The 4K iMac has a 3.0 GHz quad-core i5 processor with a Radeon Pro 555 GPU, 8GB of RAM, and a 1TB 5400 RPM hard drive for $1299. With a 3.4 GHz i5 processor, a 1TB fusion drive, and the Radeon Pro 560 GPU chipset, the cost climbs to $1499.
As far as configurations for the 5K iMac, the quad 3.4 GHz i5 model, with 8GB of RAM, a 1TB Fusion Drive, and the Radeon Pro 570 GPU chipset retails for $1799. A 5K iMac with a 3.8 GHz quad-core i5 processor, a 2TB fusion drive, and the Radeon Pro 580 GPU sells for $2299.

According to AMD, The Radeon Pro graphics found in the MacBook Pro and iMac feature the latest "Polaris" architecture with 4th generation Graphics Core Next. As with the GPUs in the series in the MacBook Pro, the new iMac GPUs feature AMD's asynchronous compute technology, updated shader engines, enhanced memory compression and new geometry capabilities.
The speeds in the new Pro 570, 575, and 580 beat out the MacBook Pro's set. The Pro 570 nearly doubles performance of the Pro 460 and 560 in the highest configuration of the MacBook Pro possible. The Pro 580 is 2.9 times faster in single precision than the Pro 560.

In comparison, the Iris Graphics 540 found on the 13-inch late 2016 MacBook Pro has a peak performance of 806 gigaflops, with the 550 in the higher end configuration coming in at 845 gigaflops. The 2013 Mac Pro desktop GPU D300 FirePro is capable of 2 teraflops per GPU, and the computer holds two. The D500 FirePro can provide 2.2 teraflops per GPU.
The Nvidia 1080ti with a native PCI-E connection for around $700 delivers around 11.3 tflop, with the $500 1080 coming in at 9 tflop. The $1200 Titan Xp hits around 12 tflop.
The 4K iMac has a 3.0 GHz quad-core i5 processor with a Radeon Pro 555 GPU, 8GB of RAM, and a 1TB 5400 RPM hard drive for $1299. With a 3.4 GHz i5 processor, a 1TB fusion drive, and the Radeon Pro 560 GPU chipset, the cost climbs to $1499.
As far as configurations for the 5K iMac, the quad 3.4 GHz i5 model, with 8GB of RAM, a 1TB Fusion Drive, and the Radeon Pro 570 GPU chipset retails for $1799. A 5K iMac with a 3.8 GHz quad-core i5 processor, a 2TB fusion drive, and the Radeon Pro 580 GPU sells for $2299.

Comments
Around 11tflops.
1. This is a consumer Mac, even the 27" model. So the majority don't really care how many giga or teraflops it does nor do most even know what that means.
2. As stated more than a few times, a large majority of customers Mac or PC these days are not interested in building their own computer or even swapping out parts. The Macs of today will last a very long time (5+yrs). There are Macs out there today that were purchased in 2009 that still work perfectly fine today. A lot of those 2009 Macs are still supported by Apple even so they can still get the latest and greatest version of macOS.
3. It seems silly to me to spend $500 on an old PC (Hackintosh) to get longer use out of it when you can take that $500 and put it toward a newer Mac. Its not all about the graphics. The flash storage alone will make a new Mac so much faster.
I was trying to make the point that we have only reached 60% of the graphics power of a year old consumer card. I understand there is another iMac coming in December and new Mac Pro next Spring. The iMac is beautiful machine, MacOS is far better for almost everything I do, but when you run in a multi-platform environment you see how underpowered Macs are at the moment. In my opinion, The iMac has too nice of screen for the graphics hardware they are putting behind it. The egpu will help, but it is not an elegant solution for a desktop, it is very nice for a portable.
I think Apples pre-announcing future hardware is the most exciting thing of the entire event, it means they may understand the need of SOME people, and really goes to prove my point. As far of iMac being a consumer machine, its really the only desktop machine they make. The Trashcan Pros, which honestly I love and wish they kept updating, are for a very very very small amount of people. I wish they had updated the Mac Mini.
Finally, it just bugs me that Apples marketing always starts out with "New graphics 200% faster than last years model" when it is so dreadful. As most people here have said, graphics speed doesn't mean that much to the most users.
I am a multi-mac household, and I don't think I'm unusual. While the latest and greatest is often attractive, my needs are modest. The oldest iMac in the house is now frozen in time on ElCapitan. It works just fine, but will soon see limits or simply die. A family member (outside my house) uses a 2005 G5 iMac. It works for the very limited things expected. I have a macmini that still works fine, but that I have no real use for (OSX 10.4 anyone?)
I weigh using my 2013 MacBook Air as a desktop in a dock with a nice display. Sure, I'd like, would use, and find value in a $1200 iMac. But $1200 is real money to most people. Maybe this could be called "modular in place over time" strategy.
As for consumers I suspect that many DO care and DO understand how a GPU impact performance. Apple has been feeling the heat so to speak with rather pathetic desktop releases so I see they WWDC as an admission from them that they have to be more responsive to consumer needs. Hopefully this isa trend that they will keep up in the future.
As for the Mini and MacPro, yep a disappointment that they haven't updated either. I actually see potential here for Apple to offer 3 different "desktop" solutions. First the Mini could be replaced by a single board solutions 1/2" thick and maybe 4" square. Then they need a midrange solution which is hopefully the rumored machine that they are supposedly working on. This machine would be big enough to take a single GPU card and support RAM expansion, frankly a cut down Mac Pro would be perfect. Then we need to see the delivery of a Mac Pro replacement that is a true "Pro" machine. Wishful thinking I know.
He didn't make anything up. The All in ones just don't do it for a lot of users.
I have always been in that small camp having had every 'openable' Mac there is I think, I updated drives, RAM, graphics cards and even CPUs on some occasions. Oddly have resisted touching my 2013 Mac Pro even though many of its parts can now be updated. Firstly it is as fast as hell still (and now with APFS internally on on it's external RAIDs) and secondly I'm saving for the middle of the range new, new Mac Pro in 2018 instead. But .... The built in 5K monitors Apple almost give away may convince me next year though when I appraise the cost benefits of a high end iMac Pro versus a new Mac Pro plus monitor. The latter would have to be pretty darn amazing t win that I suspect. Maybe Apple have a Quantum Mac Pro under wraps?
This should have been easy to release.
If they don't have a touch version of macOS ready they should really be pushing this.