I can't say it's any slower on my iMac (late 2015), seems as fast as a fast thing that's in the hurry to be somewhere important. I'm a bit disappointed as I wanted an excuse to buy a newer one (I only realised the other day that it was so old, I bought a pair of them when we moved into The Asbestos Palace, which also doesn't seem seven years ago – that said, the Mac it replaced was probably six years old, so I'm not exactly surfing the wave of technology).
Other than a few cosmetic changes (rounded corners, it's like they never went away, but I'm big fan of rounded corners) and a few dibs to the colour palette, and some new desktops, it's mostly a bit of a muchness. I've never had an issue with the upgrades (other than the mothership VPN issue, unfortunately, the main tool I use for my job can only be accessed via the VPN), they seem generally incremental improved. I'm sure they do stuff under the hood that I'm not cognizant of, but I've worked hard to be this ignorant.