This is the case even on high-end devices. Our 12-month-old Galaxy Tab is slower than a 7-year-old Pixel. Hard to understand.
Plus, they make really odd tweaks to the UI, such as adding a permanent button overlay that clashes with most hamburger icons in websites and apps. This drives novice users insane.
If you wanna ship a custom Android, at least get it right. Otherwise, just stick to stock. Sony does this really well: https://developerworld.wpp.developer.sony.com/open-source