Having used variety of brands, I would say that after few months of use like 6-12 months, Samsung tend to lose software stability more than other brands, especially when compared to Huawei EMUI and Xiaomi MIUI, that stays comparatively much stable even after 12 months.
But your case is something different. Kept Note 8 as secondary phone for a couple of months, main reason being s pen, but apart from few stutters, no performance issues i faced at all. Stayed smooth overall, but i keep majority background apps closed to prevent unwanted notifications, so that helps a lot, as majority issues in my usage are from third party apps than phone software itself.
I mostly put blame on poorly optimised social media apps that bog down Androids, be it a 10k or 100k phone. Facebook, Messenger and Snapchat are three main culprits in my experience.
