Severe Web Client Streaming Issues

We have a large event this coming Thursday where we anticipate 1,500 attendees. It is all simulive. When testing and viewing on the web client, we have been experiencing very significant issues with the video and audio. The video lags, the audio gets out of sync, we get low bandwidth or unstable network prompts. This is a very important event, and a lot of our attendees will be joining via the web client as opposed to the app for various reasons, despite our communications that the app is best for optimal viewing. Has anybody else experienced any sort of issue or has anybody raised anything to Zoom? We can't be the only ones experiencing this. We've tried literally everything. We were up until 3 a.m. today playing around with files types, tweaking any potential setting that we think could be causing the hang-up. We just tested 10 different sessions with 10 different file types, so have tested this extensively with every type of video file setting and session formats (even a live event that's screen sharing the file). We don't know what else to do and are afraid we are going to have 1,000 angry attendees on Thursday.