That does not sound like a internet issue, more like a computer issue. You can test by unplugging your internet and Zwift should work without internet.
What are your pc spec.
Win10, intel core i5, 8GB Ram, intel HD3000 graphics 1664MB ram. Have been using zwift on this PC for a long time without any issues
This is what is causing your issues, the minimum requirements are Intel HD4000.
Even if the minimum recommendation may be higher, why - when they say they “Have been using Zwift on this PC for a long time without any issues” - would there be a problem when there hasn’t been before?
Resource demands increase all the time in Zwift, even though they don’t announce it. It’s seen quite clearly at the high and very low ends. Been loads of instances of riderless bikes reported recently, for example.
Also, resource usage typically increases over time on a PC - new software installed that has something running in the background, Windows updates taking more resources.
A system may have been close to the limit without you realising, and something (apps, Windows, new Zwift stuff) tips it over the edge
Why edit your post like this, thereby stopping others learning from your experience?
Issue seem to de solved.
Closing the topic.