Sorry but I STRONGLY disagree, I am rooted and ROM'd on my Nexus 6P, and this phone just flies, ZERO lag, super smooth ALWAYS, and even better battery life due to the custom kernel. I get 5.5h to 6h Screen On time regularly with my ROM'd 6P.
Plus no adds ever, and even YouTube is add free no commercials. I have custom dark themes on here, and a deeper Settings menu to really go crazy into [H]ardcore options. With a rooted Nexus / Pixel the options are endless, and the phone isn't so plain jane compared to stock. With a good custom ROM, the default options are way more than stock, more features, like you'd get with a Motorola or OnePlus phone.
So yeah, a smoother lag free phone, significantly better battery life, and once it's all setup the way I tweak it, I'm done, and never touch the customization again, I keep it that way for months, and enjoy a great phone.