When you have a complex document and panels docked, it takes a lot of time to hide them with Tab because the position of the viewport is moved. It does not happen when all panels are floating.

1. Load Essentials Classic workspace
2. Open any document with art in it (the more complex the better)
3. Press Tab — viewport moves (and it is slow!)
4. Press Tab again to show all panels
5. Undock every panel and toolbars to make them float (including Control panel)
6. Press Tab — viewports stays in place (and it is almost instant!)

So the position of viewport should not move when Tab is pressed, in all cases.