Try it directly from your Web browser. The bitrate is lower on streaming IPTV, but it shouldn't be that bad. If you don't experience it in browser, it is your wifi and/or chromecast/smart TV setup. I thought the same the first time I rented a Google Play movie. Upped my wireless speed & got a better Chromecast setup, fixed my issue. Sure, streaming is a lower bitrate but the setup upgrade fixed any issues noticeable to my eyes.
If anything, use your Vue credentials to login to Watch ESPN. If you're still experiencing this within Watch ESPN app displayed on TV, it's your set up & not Vue.
Can anyone else confirm sports are rough to watch on Vue?
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Class of '10 - A&M Undergrad & Master's Alum