After having to do several sales meetings where video was desired, the longest being about 5 minutes, here are my recommendations:
1. Understand your network connectivity from the PC that will be sharing.
- Be on Ethernet, not wifi
- Run a speed test that includes metrics on packet loss, jitter. From Chrome, you can use this one for Webex specifically. You want no packet loss, and jitter below 30ms.
2. Ensure nobody in your home (assuming working from home) is gaming, streaming, otherwise consuming upload bandwidth at the time you are sharing.
3. PC needs to have nothing else running except the video you plan to share and Webex.
4. Ensure you will not be interrupted by other processes running, like Anti-virus or IT managed security or software updates. These can literally kill the entire experience as your CPU spikes during these times.
5. Encode the video at 720p, since this is what Webex HD video encoding is at (this was true a year ago and I will update if I learn it has been increased, but believe 1080p is for devices only). This way, Webex does not need to downscale your video if it is 1080p.
6. When you share the content, be sure to optimize for video and to share your computer audio.
You can share an embedded video in PowerPoint (local install - I'm not vouching for Office Online), just as successfully as an MP4 player.
I hope this helps solve your issue.