Hmm, have you considered background lag that is usually in applications today? One possible explanation for the freezing could be related to the browser’s resource allocation. When you switch to another application, your browser may be allocating more resources to the other application, which could cause pixel streaming to slow down or freeze. Similarly, higher screen resolutions can put additional strain on your system, which could also impact performance.
Another possible explanation is network latency. If your network connection is slow or unstable, this could cause delays or freezing when streaming graphics from Unreal Engine 5 to your browser.
I suggest you download Chrome, Firefox, and another browser to try on them all to see if it is the application, browser or network connection.