Private Sub cmdReplace_Click()
If txtReplace.Text = "" Then
 MsgBox "Please enter a replace string.", vbExclamation, "Find and Replace"
ElseIf txtFind = "" Then
 MsgBox "Please enter a search string.", vbExclamation, "Find and Replace"
Else
 rtfText.Text = FastReplace(rtfText.Text, txtFind.Text, txtReplace.Text)
End If
End Sub

Modulo

Public Function FastReplace(SSrch$, SFind$, SRepl$) As String

    Dim Src() As Byte, Dst() As Byte, R() As Byte, F() As Byte
    Dim LenF&, LenR&, LenDst&, i&, j&, OutPos&

    Const ChunkSize& = 4096

    If SSrch = "" Or SFind = "" Then Exit Function

    Src = SSrch: F = SFind: R = SRepl
    LenF = UBound(F): LenR = UBound(R)
    LenDst = ChunkSize: ReDim Dst(0 To LenDst - 1)
  
    For i = 0 To UBound(Src) Step 2

        For j = 0 To LenF Step 2
            If Src(i + j) <> F(j) Then Exit For
        Next j

        If j > LenF Then 'Found

            For j = 0 To LenR Step 2
                If OutPos >= LenDst Then
                    LenDst = LenDst + ChunkSize
                    ReDim Preserve Dst(0 To LenDst)
                End If
                Dst(OutPos) = R(j): OutPos = OutPos + 2
            Next j

            i = i + LenF - 1

        Else

            If OutPos >= LenDst Then
                LenDst = LenDst + ChunkSize
                ReDim Preserve Dst(0 To LenDst)
            End If

            Dst(OutPos) = Src(i): OutPos = OutPos + 2

        End If
    Next i
  
    ReDim Preserve Dst(0 To OutPos - 2): SSrch = Dst
    
    FastReplace = SSrch$
    
End Function


gracias