So I have a late 2013 iMac with 32GB of ram and 4GB video card with fusion drive. I am debating on selling my iMac and upgrading to the new 5K iMac maxed out.
How much of an improvement would I notice with video editing and photo editing vs my 2013 model. I am trying to decide if I should upgrade or wait a bit. I just got a 4K Camcorder so I am getting into 4K videos.
I have a top-spec 2013 iMac 27 I use for 4k video editing, and am getting a 2015 top-spec 5K iMac.
The 2013 has Fusion Drive but most of my media is on a Pegasus Thunderbolt drive array. The 2013 iMac does OK for most HD and single streams of 4k. However it really slows down on 4k multicam and becomes very laggy. This requires transcoding to proxy media, which is no problem since FCP X supports that seamlessly. If you are using Premiere it does not have integrated proxy workflow so you'd be stuck.
The 2015 top-spec model is about 20% faster from a CPU standpoint -- 14% improvement going from 3.5 to 4Ghz, and about 6% instructions per clock improvement from Skylake. I'm getting the 1TB SSD model, which will help for some things but typically H.264 video (even 4k) is not I/O limited but CPU or GPU limited.
Skylake has an upgraded Quick Sync on-chip transcoder so it can handle H.265 and VP9 in hardware. These codecs are not yet widely used but are expected to rapidly increase in the future as 4k proliferates.
We don't distribute 4k, but shoot in 4k because (1) It enables significant recomposition by the editor without loss of resolution in a 1080p project, and (2) Each 4k frame is an 8 megapixel still so frame grabs are very useful, and (3) 4k downsized to 1080p is slightly better quality than native 1080p.
I'd recommend getting SSD because your 4k content will not fit on the boot drive even if it was 1TB. Some of our 4k cameras are sending 90 GB per hour to the output. Proxy media (essentially required for 4k multicam on iMac) increases this by about 60%, plus your regular media is still there. So one hour of 4k material by two 200 mbps cameras would be 2 * 90GB/hr * 1.6 = 288 GB. And that is with no render files, no scratch files, no optical flow files, etc.
For practical 4k work of any significant size, it's far beyond the capacity of a 1TB SSD boot drive, yet you don't want to bog down a 3TB Fusion Drive boot partition with that, either. It essentially requires a high-performance external drive, typically an array, which means you may as well get the benefits of an SSD boot drive.