A trick that can also help: the removal of " useless " constraints or more news.
For example, a part that you would have removed from your assembly but whose constraint remains unresolved (but why on earth does Solidworks do that?)
Macro Identification and Removal of Faulted Constraints - Macro - myCAD Forum
I would even say in the case of unnecessary constraints, remove parallelism constraints in sketches as well as in assemblies when possible. Or find another way to coerce.
Parallelism is often redundant with other constraints or dimensions.
Do a test, go into a sketch, enable the sketch feature " Show/Delete Relationships " and as soon as you see parallelism, delete them. If the sketch is not constrained, add a constraint of the collinearity type or a dimension or direction. You're going to delete a lot of them.
It is important to know that constraints form loops that suppress degrees of freedom and often orientation constraints are redundant to positioning constraints. You have two constraints instead of one. And when your sketch is large or many constraints accumulate, SolidWorks gives you a sketch on constraint for no reason. Well, yes, but it's hard to control!
In assemblies it's different as @coin37coin says, the constraints that have not been removed are due to the dialog box that asks you beforehand if you want to delete the elements related to a part that you are excavating. If this box has been unchecked, then next time it will keep the constraints and put them in a delete state.
The skeleton principle eliminates 2 thirds of the constraints of an assembly, except for the screws
But that doesn't take away from the fact that you have a high-performance PC on SW
I don't know about you, but I have the strange impression that @FRED78 has a lot of skeletons in his closet! 
I work for a company that wants to work on SW with skeletons.
After + 20 years on this software I have seen pretty much all the bugs and methods that work (but I am open to learning).
But for the past three years, when I joined this company. I have some clever people who play with PCs and certainly the settings of my software and hardware 
.
And I try to maintain a method on the project and on the software. That said, my PCs (yes I have several) don't all work the same 
, which is the last straw for a company that provides standardized 
PCs.
So I discovered new BUG or to paraphrase @sbadenis I dug up new skeleton 
.
1 which I liked much more "we don't know about this bug, please indicate the error code". I admit I had a lot of fun SW asks you for the BUG number
But I take it with a lot of hindsight 
, even if sometimes some employees have a curious way of working or collaborating for their clients, still skeletons 
. Here it allows me to show these same collaborators the explanations, because I gave them this site as a reference
After all, I don't hide behind my Psquedo and I assume perfectly what I say, with all due respect to some.
Another point that considerably weighs down the assemblies, the constraints in red (the worst of all that make the software recalculate), sketch them with freedom (-)
Another point, the unhidden sketches or plans, points of origin...
Even if the assembly mode hides the sketches (display show hidden) they are still loaded when the assembly is opened unless hidden in the part (cf a Visiativ trainer)
Some of these points combined in the example above make an assembly lag considerably.
The transparency of some parts also makes you row a lot, as well as big repetitions (like wire mesh), the only type of part where I systematically add the blocking bar...
Edit: @FRED78 I also have collaborators who bring together all the above points in the same assembly and when I go back to an important assembly it sometimes takes me an hour or 2 to go back to all these points.
On the other hand, much less crash afterwards and a considerably faster opening...
@sbadenis
I'm from school where I hide everything and only show what I want to show.
This saves me from having a bag of knots when I want to show a single sketch for example.
But to go in your direction, I have one, who gave me axes on all these oblong sketches! no but Hello !! , but this same person puts axes on holes with the same tool , it's psychology. At the same time I laugh about it...
But my PC problems go far beyond SW, I've been using this software for many years. I had the opportunity to work on LNG carrier bridges (piping) with the routing module and fully parameterized assemblies. Suffice to say that the assemblies reached significant weights and a just incalculable number of parts.
Now I have a lot of assembly but nothing insurmountable.
Now he just changed my PC m^me config all the same, even under a layer of sea..., and curiously it works (Standard exchange). We are more in: the exchange of who will have the last words