There are two settings in Options that can improve navigation and speed up the Walk Through mode for long range data (for example .FLS, .BTX, .PTX, .ZFS, etc).
To change these settings, access the Options menu by the Application Button > Options (or use CTRL +X).
1) Options>Display>Navigation>Walk Thru: Move the Keyboard and/or the Scroll Wheel sliders all the way to the right.
2) Options>Graphics>OpenGL>Use Display Lists: Check the Unordered Points and Ordered Points.
Note: With these options selected the navigation is faster, however memory usage on the computer increases. Use this option with caution in regards to how much memory is available on your computer and the size of the point cloud.