Looping through individual cells in a range can be slow. It is better to read the range directly into an array, modify the array, and write it back out:
vb Code:
Sub RangeMod()
Dim nrow As Integer, ncol As Integer, i As Integer, j As Integer
Dim mf As Double
Dim a As Variant
mf = 1.15
nrow = 27
ncol = 8
a = Range("D5").Resize(nrow, ncol) 'read range into array
For i = 1 To nrow 'loop through array
For j = 1 To ncol 'loop through array
a(i, j) = mf * a(i, j) 'modify array elements
Next j
Next i
Range("D5").Resize(nrow, ncol) = a 'write array back to range
End Sub