To get something to show as you do the looping you need to have a call to "DoEvents" - otherwise nothing will happen until all the code has run.
Also X is being passed to that function (as the horizontal position of the mouse pointer), but you are re-using it as something else - so best to use a different name for that variable (doesn't make any difference in this case however):
Private Sub Label1_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Single, ByVal Y As Single)
Dim i As Long
Dim T As Double
For i = 10 To 1 Step -1
ActivePresentation.Slides(1).Shapes("Rectangle 3").Fill.ForeColor.RGB = RGB((i * 10), (i * 10), (i * 10))
ActivePresentation.Slides(1).Shapes("Rectangle 3").TextFrame2.TextRange.Characters.Text = i
T = Timer
Do While Timer - T < 0.5
Loop
DoEvents
Next i
End Sub
Note, however, unless the timer wait value is quite a lot more than 0.5, not every decrement of the counter will be shown in the text box. So this is not a complete solution to your problem.