About 1-2 months after its launch in 2014 I decided to purchase a 5k iMac with specifications: i7 4.0 Ghz, 1 Tb fusion (128 GB solid state included at that time), m290x 2GB, and 24 GB of ram. The user interface, and overall zippiness of the system dragged my molasses in Yosemite, and only slightly improved in El Capitan. Needless to say I was pretty disappointed with the system... until today. This morning I installed the macOS Sierra Golden Master, and it's a night and day performance difference. There isn't any noticeable lag in the user interface, and finder tabs populate almost instantly. Chrome and Safari tabs populate noticeably faster. While it would have been nice to see this level of performance on day #1, I now feel like my purchase of the 5k iMac is less of a waste than I did before. Highly recommend you install macOS Sierra.