How do I reduce lag without deleting anything?

How can I compress the data as much as possible without deleting any object?

From you image alone, I don’t see anything that should cause lots of lag. I am thinking it’s probably not the polygon count, unless one of the objects is very high-res.

A few things to check as a start:
-Do you have lots of individual objects, like thousands?
-Are there lots of constraints or parented objects?
-Are there physics simulations?
-Are there textures with massive resolution?

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at least a few hundred objects and a lot of parenting

A few hundred objects should not be a problem unless you have a weak computer.

The parenting could be a cause of lag, especially if it’s chained with multiple levels of parents. Try making a backup of the scene and unparenting everything, to see if that’s the cause.

If it doesn’t make any difference, then I would look at modifiers instead. Are there objects with subdivision, remeshing, array or any modifier that deforms objects?