I wired up a grid to an AjaxProxy and store with "buffered" set to true, page size 100 rows. When the grid loads, if "forceFit" is true, there will be a 1-2 second delay between the unmask of the grid and the appearance of the scroller.
Removing "forceFit" from the grid config makes the delay between unmask and scroller appearance negligible.
Update: the 1-2 second delay is in Chrome. In IE8 it's nearly 10 seconds.