This happens only if there are many columns outside of the viewport (a scrollviewer is shown), with their TabStopMode property set to Skip.

In this case, the navigation needs some time in order to get to the next row and select the first available cell.

A possible workaround for this would be to implement a custom Keyboard Command Provider, and to replace the MoveNext command with a custom one, that moves the current cell, by setting the CurrentCellInfo property of RadGridView.