I have noticed that when I leave a browser tab in Chrome with CesiumJS running and return to that tab after browsing other pages, opening of that tab has noticeable lag (~2 seconds).
I have latest Chrome (88.0.4324.96) on pretty powerful MacBook Pro (15-inch, 2019) and MacOS Catalina.
This doesn’t happen e.g. on Safari browser on Mac. Could it be that Chrome is putting some worker code to sleep & restarting it when tab is opened after some time? If so, could some kind of timeout be added so that it wouldn’t interfere with opening of the tab.
I can reproduce this with the Sandcastle basic demo page running at: https://sandcastle.cesium.com/