The warning isn't the end-all-be-all unless the system is actually slowing down (you would see an audio or video delay from the attendee side). Reference doc: https://support.goto.com/meeting/help/how-do-i-change-my-webcam-background
Do you know what operating system the affected participant is using, and if they rebooted right before joining the GoToMeeting?
We have over 50% of our users experiencing this as well when they use the GoToOpener to join a meeting. Users are on Dell 7480, 7490 or 5501, newer laptops running Windows 10. I contacted Tech Support and was informed the message was a new "feature" to let attendees know when web cam was using too much of the CPU. Our observation is that users who are in and out of several meetings a day have multiple instances of GTM running in the background, consuming higher than expected processor resources. The issue is not the error message, but the underlying cause of it. Using the web client to join the meeting does not produce the same error. However, our users tend to do alot of screen sharing and sharing of keyboard and mouse control.