How does DWA Planner calculates speed command?

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When I used the dwa_local_planner my robot usually moves with 0.3 m/s and it never reaches the maximum speed. I tried it in an open corridor when the robot is long away from the obstacles and goal position. I guess in this condition robot should be moving at the maximum speed but it is not. So what are the parameters that dwa planner considers to calculate speed?

I am using the following parameters for speed limitation:

  min_vel_trans: 0.05
  max_vel_trans: 0.4
  acc_lim_theta: 10.0
  acc_lim_trans: 5.4

  forward_point_distance: 0.3
  acc_lim_x: 5.4
  acc_lim_y: 0.0
  max_vel_y: 0.0
  min_vel_y: 0.0
  max_vel_x: 0.4
  min_vel_x: -0.4
  max_vel_theta: 0.4
  min_vel_theta: 0.18

Here is my simulation time parameters:

  sim_time: 5.0
  sim_granularity: 0.025 
  angular_sim_granularity: 0.1
  vx_samples: 3.0
  vy_samples: 0.0
  vth_samples: 30.0