How to temporarily fix the constant lag:
The performance issues seem to be related to the Symbols panel. Either due to the amount of symbols you have, the complexity of them or the number of times a symbol is used in your artboards – or maybe all three. I check all three boxes, but experienced a drastic improvement in performances after removing all symbol from the Symbols panel.
While it's probably not ideal for some to delete all symbols from the Symbols panel, this does not unlink the symbols already used in artboards from one another. So you will still have the benefit of changing one symbol in an artboard, and see that change reflected in all instances of that symbol used in any other artboard. If you need another instance of that symbol, you just copy it from one of your artboards rather than dragging it from the Symbols panel.
Deleting all symbols from the Symbols panel can take a while as it's currently only possible by right-clicking every single symbol individually and choosing "Delete Symbol", but you will see the lagging gradually disappearing for each symbol you delete.
I also noticed that the load time of opening the AD file after deleting all symbols was reduced more than 50%.
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Correction dec. 3: After continued work with this approach, it seems like deleted symbols from the Symbols panel are in fact unlinked on all artboards. I do, however, still have files where all symbols are now deleted, but are still linked in my artboards. I don't know why this is.