Nope, it's Sling. Use the AMC app? Perfect HD. Use sling? Won't load or looks like trash. WatchESPN app? Perfect. Sling? Trash. I have 100mb Comcast and their router, I also have an additional $150 AC router I use because it works better. On my MacBook the app crashes consistently, wifi or wired. The iPhone and android apps work fine for the most part but I'm not watching TV on my phone at home. The Roku works better than my chromecast but it still frequently slows down and loads. The biggest issue is that as soon as I try to send anything to my chromecast, it doesn't want to work. Wifi or hard wired. Sometimes it loads and is fine, sometimes it shows I'm about to cast but nothing loads. It's just been nothing but infuriating and they're customer service has no answers, they just blame you. They blame you and don't even have any technical know how to try and work through what the issue may be. When sling works it's awesome, which is 65-70% of the time. My thought is that they just have incredibly poor apps/internet integration and that trying to send anything to another device is just unstable.
There isn't anything wrong with ISP/router though. I have a 4K OLED and I use YouTube 4K, Netflix 4K, Vue, HBO app and I have ZERO issues. No loading, no buffering, no freezing,no pixelation, nothing.
If it works for you and others, great. It just hasn't been kind to me.