Why? Because of this excellent thread?
http://www.aivosto.com/vbtips/stringopt.html#whyslow

One thing (apart from the optimization issues on string functions) I've encountered is needing to prepend VBA. on all of these kinds of functions, because of compile issues in various environments (so that VBA.Left compiles correctly, when Left would cause a compile issue), but other than that... what has Paul said?