Just got the new iMac 27 (580, 2TB Fusion) - and was doing a test on 1080 video that would play choppy on my old 2009 iMac. Sadly, this same video is doing the SAME thing on this brand new computer that's not only supposed to handle 1080, but 4K as well. It's a MKV file, currently getting converted into an MP4 on Handbrake. Why would this occur on this machine? I bought this computer specifically to have smooth editing and playback for HD videos, and it's already acting up. I am tempted to return this but wanted some advice before I call Apple Care. Could the choppy playback be a result of me filming it years back? Or perhaps because my computer still only has 8GB of RAM and I have not added my new RAM yet because it has not arrived. It plays fine until I attempt to fast forward it to a different part. It's only a 20 minute video. Any advice would be appreciated. UPDATE: I transferred it through handbrake, and it plays smooth as butter in MP4 format. Still, strange the raw MKV file was choppy. Not sure if that is because of the specific file, or something else.