Access 2010: UDF in calulated query field runs painfully slow over network

I have a UDF in a calculated query field that returns the names of one or more people connected to each record. It runs great in my laptop but when I run it over our network (front-end on my PC and back-end on server) the form can take up to 12 seconds to open. If I remove the calculated field from the query (troubleshooting) the form opens immediately. Any idea how I can speed this up or do this a different way?

I have attached a snapshot of the query. Here is the function:

Function GuestNames(lngRoomID As Long) As String
 
  Dim dbs As DAO.Database
  Dim rst As DAO.Recordset
  Dim strSQL As String
  Dim strGuestNames As String

  Set dbs = CurrentDb()

  strSQL = "SELECT tblCustomers.LastName " _
          & "FROM (tblCustomers INNER JOIN tblTourBookings " _
          & "ON tblCustomers.CustomerID = tblTourBookings.CustomerID) " _
          & "INNER JOIN tblCustomerRooms " _
          & "ON tblTourBookings.TourBookingID = tblCustomerRooms.TourBookingID " _
          & "WHERE tblCustomerRooms.RoomID = " & lngRoomID & " " _
          & "ORDER BY tblCustomers.LastName;"

  Set rst = dbs.OpenRecordset(strSQL, dbOpenDynaset)

  If Not rst.EOF Then
    rst.MoveFirst
    Do While Not rst.EOF
      strGuestNames = strGuestNames & rst!LastName
      rst.MoveNext
      If Not rst.EOF Then
        strGuestNames = strGuestNames & ", "
      End If
    Loop
    GuestNames = strGuestNames
  Else
    GuestNames = "Enter Name(s)"
  End If
 
  rst.Close
  Set rst = Nothing
  dbs.Close

End Function