For .NET and Python programs, it is much more efficient to build a linear or quadratic expression by modifying an existing expression rather than repeatedly creating new expressions. So for .NET, use the AddTerm/AddTerms methods instead of the overloaded operators, and for Python, use the += or -= operators rather than creating new expressions.
For Python, try Model.addVars() to create a sparse tupledict of variables, then use the select(), sum() and prod() methods to iterate only over matching variables. This is illustrated in the netflow.py example.
Finally, if you wrote your program with an old Gurobi version that required the update() function, try removing that function and running your program with the latest version.