Each time that you use the ampersand ("&") operator, Visual Basic allocates memory for the new string and copy the old string into the new string. If you concatenate a lots of small strings into one big string, the operation is quite ineffective because Visual Basic allocates memory, copy, and deallocates memory a lots of times !
You can solve this problem by explicitly allocating big chunks of strings when it needed.
The following class lets you easily and efficiently concatenate strings. It automatically allocates more space when it needed, in chunks of 1000 characters. You can download the sample project below, and see that string concatenation with this class might be much faster than doing it with ampersand operator.

'String Concatenation Class ' 'Written By Nir Sofer 'http://nirsoft.mirrorz.com ' 'The following class can help you to Concatenate 'small strings in more efficient way than 'the standard VB "&" operator does it. ' 'In order to Concatenate strings with this class: ' '1. Create a new instance of this class. '2. Call the Add method to add small strings into the main string. '3. Call the GetStr function in order to get the accumulated string. Private strText As String 'The stored string Public lngAllocated As Long 'Number of allocated characters Public lngUsed As Long 'Number of characters in use Public lngAllocSize As Long 'Number of characters to allocate every time Private Sub Class_Initialize() 'By default, allocate 1000 characters each time. 'You can "play" with this value in order to get the best efficient string allocation for you. 'If you increase this value, the string Concatenation operation 'can be a little faster, but it'll waste more memory. 'If you decrease this value, the string Concatenation operation 'can be a little slower, but it'll waste less memory. lngAllocSize = 1000 End Sub Public Sub Add(strAddString As String) Dim lngLen As Long Dim lngToAllocate As Long lngLen = Len(strAddString) If lngLen > 0 Then If lngUsed + lngLen > lngAllocated Then 'Calculate the characters to allocate. lngToAllocate = lngAllocSize * (1 + (lngUsed + lngLen - lngAllocated) \ lngAllocSize) 'Allocate more space in the string. strText = strText & String$(lngToAllocate, " ") lngAllocated = lngAllocated + lngToAllocate End If Mid$(strText, lngUsed + 1, lngLen) = strAddString lngUsed = lngUsed + lngLen End If End Sub 'Returns the accumulated string Public Function GetStr() As String GetStr = Mid$(strText, 1, lngUsed) End Function

Download sample project