VBA Function slower than Excel Formula

I've replaced the following horrendous Excel formula (values and names have been replaced):

=IF(AND(SourceTable[@Field1]="Value1",SourceTable[@Field2]="Value2"),"Result1",
    IF(SourceTable[@Field2]="Value3","Result2",
        IF(AND(SourceTable[@Field1]="Value4",SourceTable[@Field3]="Value5"),"Result3",
            IF(AND(SourceTable[@Field1]="Value6",OR(SourceTable[@Field3]="Value7",SourceTable[@Field3]="Value8")),"Value9",
                VLOOKUP(SourceTable[@Field1];MatchingTable;2;0)))))

With a VBA Function so it would be easier to read and supposedly faster:

Function CUSTOMFUNCTION(oRngField1 As Range, oRngField2 As Range, oRngField3 As Range) As String

    'If imbrication
    If (oRngField1 = "Value1" And oRngField2 = "Value2") Then
        CUSTOMFUNCTION = "Result1"
        Exit Function
    ElseIf oRngField2 = "Value3" Then
        CUSTOMFUNCTION = "Result2"
        Exit Function
    ElseIf oRngField1 = "Value4" Then
        If oRngField3 = "Value5" Then
            CUSTOMFUNCTION = "Result3"
            Exit Function
        ElseIf (oRngField3 = "Value6" Or oRngField3 = "Value7") Then
            CUSTOMFUNCTION = "Result4"
            Exit Function
        End If
    End If

    'Vlookup if no match before
    Dim sTableMatchingFieldLookup As String
    Dim sTableMatchingFieldResult As String
    sTableMatchingFieldLookup = "MatchingTable[Field1]"
    sTableMatchingFieldResult = "MatchingTable[Field2]"
    CUSTOMFUNCTION = Range(sTableMatchingFieldResult)(Application.Match(oRngField1, Range(sTableMatchingFieldLookup), 0))

End Function

I get the same result as the function however it does it two times longer than the formula when working with 2k rows. Why isn't the code faster than the Excel formula? Can it be improved? The file is expected to grow so it means the code will be of no use.

Edit: Using more arguments improved the macro, see my response below.

I'm assuming that names have been changed to protect the innocent, but just in case they've not been...

These are terrible names. All of them.

• Function names should have verb-noun names and be PascalCased. So instead of CUSTOMFUNCTION should be GetCustomMatch (or something like that. I can't say because you've ommitted any actual names...)

-
Parameters should be camelCase and tell me what kind of data
should be passed in and NEVER should they be numbered the way
this is. So, maybe this instead (again, I have to guess because I
have no clue):

Function GetCustomMatch(LookupRange As Range, lastYearsSalesRange as Range, iJustDontKnowWhatThisIsRange as Range) As String

-
Don't use hungarian notation. Just... please don't. It should be
killed with fire and buried out back.

Other Stuff

• Some of your "Values" are used in multiple places. Store them in a

constant and use that constant anywhere they show up. (Of course,
use meaningful names for them.)

• I have a feeling that sTableMatchingFieldLookup and ...Result should be actual ranges instead of strings. String typing is bad. If you insist on keeping them as strings, store the values as constants.

• You should be able to pick up some performance by setting Application.Calculation = xlCalculationManual. This Stack Overflow answer has a pretty good example of what you're trying to do.

Things you did right

• You exit early if you've already found a match before performing the lookup rather than nesting if statements.

StackExchange Code Review Q#56023, answer score: 14