This is the normal behavior of Sw on large assemblies: removal of small parts and blocks containing large ones as long as you move the mouse. SW only does the calculation once the view is 'stationary'.
You have to rummage through the assembly options and large assemblies to drive from part combination SW uses this degraded mode.
Check this: options/assembly/opening a complex assembly.
We had the problem where when installing the new version the component number was set to 63 (or something like that) and suddenly we had the same kind of problems as you.
We set it to 3000 for the light mode and unchecked the management of large designs (useless with us)