Blurry Slides & Video During Zoom Webinar

Were these all people working at your main office, or were some remote?

It sounds like network bandwidth/congestion issues, either way.  At home/remote, you're at the mercy of your internet provider, and if you're on a cellular connection, same issue.

Some people think that having everyone in the office attend via Zoom means no bandwidth issues... but keep in mind Zoom is then feeding one video stream to each of your participants.  All it really takes is one person Screen Sharing with a 4K monitor, and all of a sudden your bandwidth needs can quadruple... and some people are likely to experience issues.  (Screen sharing is sent to all participants in the native screen resolution!)

So not sure what your specific issue was, but there's some ideas to think about.

Webinars with Zoom have been a constant source of frustration. We are an enterprise customer that spends over 200K per year. The webinars are atrocious, even after repeated high-level support calls, account managers, and other avenues. The quality is blurry and there are over 1000 people in the call in most cases. We are looking for replacement at the moment.