hectorslam + navigation stack : obstacle inflation problem

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Hi, I'm trying to use hectorslam in conjuction with the navigation stack for automatic map exploration. My robot has odometry and laser sensors. Hectorslam works well (with joystick teleop by exemple) but I have some problems with the navigation stack. First of all, my robot has some problem to move because some wrong obstacle detection. Here a screenshot of my rviz output that shows inflated obstacles. There is inflated obstacles in the unknown areas. I don't understand why :

Following the navigation tutorial it shouldn't have inflated obstacle in unknown spaces :

Here, my navigation configuration files :

psykokwak@robot:~/ros/workspace$ cat arobot/config/*
TrajectoryPlannerROS:
  max_vel_x: 0.45
  min_vel_x: 0.1
  max_rotational_vel: 1.0
  min_in_place_rotational_vel: 0.4

  acc_lim_th: 3.2
  acc_lim_x: 2.5
  acc_lim_y: 2.5

  holonomic_robot: false
  meter_scoring: true



obstacle_range: 2.5
raytrace_range: 3.0
footprint: [[0.2, 0.2], [0.2, -0.2], [-0.2, -0.2], [-0.2, 0.2]]
inflation_radius: 0.50

observation_sources: laser_scan_sensor

laser_scan_sensor: {sensor_frame: /laser, data_type: LaserScan, topic: /scan, marking: true, clearing: true}




global_costmap:
  map_type: voxel
  global_frame: /map
  robot_base_frame: /base_link
  update_frequency: 1.0
  static_map: true





local_costmap:
  map_type: voxel
  global_frame: /odom
  robot_base_frame: /base_link
  update_frequency: 5.0
  publish_frequency: 5.0
  static_map: false
  rolling_window: true
  width: 3.0
  height: 3.0
  resolution: 0.02

An other problem I have : My computer is an Intel core i5 and the "move_base" process take 40/50%... For comparaison, the "hector_mapping" process take only 10%...