More efficient (faster) sub-String method?

I have a piece of code that works very well but takes a long time. 1s per instance to be exact. My data is going to approach 3000 records so that translates into almost an hour to run through the entire data set.
I am looking for code that will produce the same results but in much less time. Is it possible?

Basically I have a Column that contains names with English and French acronyms separated by [space]/[space]. I need just the English acronym.

Group Name:
ABCDEFG / ABCGDF
XYZ / ZXY
ABC123 / CAB123

Changes to....

Group Name:
ABCDEFG
XYZ
ABC123

This is the code I am using:

    Dim SupGrpRange As Range
    For Each SupGrpName In Range("A3:A" & LastRow)
      a = SupGrpName.Text
      b = InStr(1, a, " /")
      If b > 0 Then
        SupGrpName.Value = RTrim(Left(a, b))
      End If
    Next