Lets say we want to perform the following concatenation:

Dim i As Long
Dim s As String
For i = 1 To 100000000
	s = "A" & i & "B"
Next i

This takes approximately 6000 ticks. The faster approach, yet more complex functionality would be as follows:

Dim sourceLength As Long
sourceLength = 1
Dim source As String
s = "A B"

Dim i As Long
Dim s As String
For i = 1 To 100000000
	source = CStr(i)
	If Len(source) > sourceLength Then
		sourceLength = Len(source)
		s = "A" & Space$(sourceLength) & "B"
	End If
	Mid$(s, 2, sourceLength) = source
Next i

Which takes approximately 3700 ticks, a saving of nearly 40%.