It's not so much that it slows down but that it is unpredictable, going by my husband's and my experiences so far, mostly with the S8+. He's a business "power user" with hundreds of apps, while I'm more narrowly focused and very cautious. But we've both run into unexpected lags and weirdness out of the blue. He's had unexpected battery drain. I have the really annoying animation stutters and lag on my home screen, going to it and from it for any app. We've both had the camera "stick" or lag on occasion.
That does not negate the many thoughtful features that Samsung did indeed bake into their version of Android. It just means we have to make allowances that these issues crop up and know that how Android works for us isn't necessarily how it works in a more stock form on other phones. Android runs smoother on my HTC 10 that's closer to stock, but has fewer "amenities" than is offered on the Samsung version.
My husband still quite likes his S8+ and isn't considering any other Android phones and still retains an interest in the Note 8. I'm sort of interested in the S8 Active but also curious about the HTC 11 and the upcoming Pixels. However, my S8+ isn't going anywhere anytime soon. I smooth out the hiccups as they come up.
I must say that Android on the S8+ is still quite good. It's very stable in the sense that I can't remember offhand experiencing any apps crashing. My apps on some of the more recent versions of iOS 10 crash quite often. iOS 10 doesn't have any stutter or slowdowns that I've seen, though.
