filters demo runs very slow.

[lianghu3124]

I run filters_demo.launch demo on my desktop computer, which has 16GB RAM and i7-8700 CPU and GTX-1060 GPU. I used the default image terrain.png
My running environment is ubuntu16.04LTS and ros-kinetic.
RVIZ shows grid_map very slow， and offen goes frozen.
When I check the cpu and gpu usage, it does't seem to be overloaded.
If I use a smaller image which is 1/4 of the terrain.png, it can show up.

[skohlbr]

This reads somewhat similar to #167 (i.e. the rviz plugin becomes extremely slow with increasing map size).