A StringBuilder class based on the work of @Blackhawk, published under CC-BY-SA on Code Review Stack Exchange.

Concatenating strings in a loop is a very inefficient process. Using a StringBuilder for concatenating a large number of strings makes the code much more efficient and noticeably enhances performance.

• 1K strings; both concat & builder are pretty much instant
• 10K strings; both concat & builder complete in ~0.05 seconds
• 20K strings; very similar performance, ~0.1 seconds for both
• 50K strings; concat ~0.65 seconds, builder ~0.2 seconds
• 100K strings; concat ~2.7 seconds, builder ~0.4 seconds
• 1M strings; concat ????*, builder ~4.9 seconds

_{*Gave up, killed the process after 10 minutes.}

With the StringBuilder class added to your VBA project, use the New keyword to create a new instance of the class:

Dim sb As StringBuilder
Set sb = New StringBuilder

This initializes the builder with a default initial capacity. To specify an initial capacity and/or initial content, or to create an instance of the class from a VBA project that's referencing the VBA project the StringBuilder class is loaded from (e.g. an add-in), use the Create factory method off the default instance instead:

Dim sb As StringBuilder
Set sb = StringBuilder.Create("test", 32)

Use the Append method to, well, append a string to the builder; use the ToString method to retrieve the string:

Dim sb As StringBuilder
Set sb = StringBuilder.Create

Dim i As Long
For i = 1 To 100000
    sb.Append "Test"
Next
Debug.Print sb.ToString

Instance members cannot be invoked from the default instance.