Why would a faster internet connection cause problems only with Roku devices?
I have a ticket open with the new fiber provider but no answer yet.
The speed won't make a difference. In fact, you can stream 4K video with 25 Mbps. Your higher speed won't help nor hurt.
The clue, I believe is that you have a different ISP. Contact them about why your Roku is having problems with their service. They could be throttling the servers to which Roku connects. You see, it's possible, and for some services, likely (or even definite) that they have different servers for Roku than for Google/Android TV devices, or Amazon devices, or browsers, or iPads, and so on. If the new ISP is throttling the servers to which Roku connects, you'll have a problem as your describing.
DBDukes Roku Community Streaming Expert Note: I am not a Roku employee.
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@jself No home user or even most businesses need 1GB internet speed. Nothing requires it. Even 4K streaming only requires 25 Mpbs. Some Roku devices are only 10/100. So even if you have a 1GB internet it will only use 100 Mbps. Even with a family of 4 500 Mbps is plenty. 1GB internet is a gimmick.