This is normal behavior of SolidWorks to unload the display.
Complex assembly mode activates when there are more than 500 parts (default).
If there are no slowdowns in the design, you can disable this mode or increase the number of parts, in the "Tools > Optionsmenu and click on Assemblies".
Help topic:
"When Complex Assembly mode is active:
Hide all planes, axes, sketches, curves, annotations, etc.
Select Hide All Types from the View menu . You can override this option by turning off Hide All Types in the View menu, and then choose whether to show or hide the individual types."