I'm not surprised, with all the bloatware and crap they put on Samsung phones it will only be a matter of time before the S9 becomes laggy. The iPhone may be expensive but i've never had an issue with it and Apple are able to maintain both the software and hardware which is why i'll stick with iPhone.
Let's be honest, as much as Apple is close to my heart, I have to admit that iOS 11 is a disaster. Yes, updates have fixed much of bugs, but Apple should have never released such problematic OS. I hope, and I am very convinced, that iOS 12 will show once again why there is nothing, and there will never be OS like iOS, and iOS 11 will very soon be just an ugly memory. I really do believe horrible bugs in iOS and security flaws in macOS have been the wake up call for Apple.