Correct.
If by tabs you mean dashboards, no only the visible widgets are rendered.
To debug performance problems, open the developer tools in Chrome, go to the Network tab and refresh the page. Check the Waterfall column.
If there’s a request, for instance “items”, taking a long time to complete (i.e. a long bar, disregard “events” because it’s a long-running SSE connection as explained above), that might be the cause of your dashboard taking a long time to appear.