Economic transmission planning requires intensive production cost simulations. The long simulation time however presents practical challenges and limits the scope of analyses. This paper therefore investigates transmission constraints, a major contributing factor to simulation time, and proposes a framework to identify and remove transmission constraints that are least critical to simulation results yet can reduce simulation time. Numeric tests using MISO planning models have shown the proposed method can achieve 10∼15% savings in simulation time with acceptable impact to simulation result accuracy. Applying the proposed method may save thousands of computing hours per planning cycle.