ICart.ApplyDiscounts follows secondary relationships of the products & times out

We sell products (services) for vehicles, and created vehicle structure in our catalog.

After that we linked all our products with the vehicles.

What is happening next, when a product is in the cart, EPI reads all vehicles linked to that cart.

This is how we link the service product variants with the vehicles: 

           IRelationRepository.UpdateRelation(new NodeEntryRelation()
            {
                IsPrimary = false,
                Parent = vehicle,
                Child = service
            })

The problem that the cart calculation stopped working completely - a service (say wheel alignment) can be linked to 10,000 vehicles.

when this code executes: 

ICart.ApplyDiscounts(IPromotionEngine, new PromotionEngineSettings())

It never exists the line. If I try to step into the line, I'm hitting setters on vehicle node which suggests it is reading the nodes from the catalog, and the only reason it can read them from this line of code is because they are linked.

The issue is happening when there is an active (any) promotion in the system.

I'm now wandering how I can stop cart / promotion engine to traverse the linked entities, as it completely throws the release off track and putting me on fire.

Please help!