Neil has cable based internet with 300 Mbps down, which should be great for streaming. But when he tries to use live TV, he finds the buffering makes streaming unwatchable. It's not the same with video on demand, though. Leo says that 300 Mbps is the "ideal" rate and it's always "up to" that amount. Leo recommends running a speed test from SpeedTest.net to see what he's really getting. DSL Reports has a really accurate speed test as well. He should plug directly into his modem and run the speed test. If he gets over 50 Mbps consistently, then he should have plenty of bandwidth for live streaming. Services like Netflix will adjust the stream to his internet bandwidth, but Sling doesn't do that.
Leo says he can also try the Roku TWiT app, as well as the live stream on You Tube. Also the app he's using may have issues. Another possibility is Wi-Fi congestion.