Make an assembly lighter

You have a couple of options, depending upon your situation.

1 - When you open the assembly, you can use the Open Subset button and use the checkboxes in the model tree to only activate the assemblies that you want to see.  This is essentially an on-the-fly simplified rep.

2 - For ongoing project that you will be repeatedly be opening and closing, use pre-defined simplifed representations.  There are many type of these, so I would encourage you to investigate it in the help menus.  For our machines, I commonly create custom reps of the sub-assemblies, using rules to suppress fasteners, bearings, springs, and some small parts.  I then select these simplified reps in the main assembly's simplified rep.  This speeds loading of the main assembly and significantly reduces the hidden-line generation times in the main assembly drawing. (Helical cuts and protrusions just clobber HLR times)  You can even make simplified reps of individual parts, defeaturing them for use in upper-level assemblies. 

Re: Help with large assemblies and structures

You may also set the config.pro option..

open_simplified_rep_by_default  = yes

So when you open an assembly, a dialog will appear so you choose which simp rep to open it in. You can choose a system-defined simp rep (such as graphic rep) or user defined rep that was previously created.

I assume you opened the assembly as a master rep, you could try opening it up as either a geometry or graphical rep. Both of these are lighter and will open faster than the master rep. You will be limited in what commands can be performed on the graphical rep.