Symfony 4 Sonata FormBuilder skip loading some relations

Question

I've inherited an older project to provide maitanance. I don't have Symfony experience, I am coming from Laravel/Yii experience.

I've tracked down a certain ajax call that takes 30 seconds to this segment:

$formBuilder = $admin->getFormBuilder();
$form = $formBuilder->getForm();
$form->setData($subject);// <- this call takes more than 9 seconds
$form->handleRequest($admin->getRequest());

The #subject in this context is Entity\Product which has lots of relations. I debugged the code until I understood that this code block loads ALL the relations of the product, such as articles, documents linked to the product, related products, which are a long list to load. The output is more than 24MB in size.

Now that I am newbie to Symfony I don't know what to optimize, or how to start reducing this load on this script.

But I know, that I want to skip certain relations, so they should not be loaded.

For example this is the document definition

/**
     * @var ProductDocument[]|Collection
     * @ORM\OneToMany(
     *      targetEntity="ProductDocument",
     *      mappedBy="product",
     *      cascade={"persist"}
     * )
     * @Sonata\ListMapper
     * @Sonata\Order\ShowMapperExclude
     * @Sonata\FormMapper(
     *      with="ProductDocument",
     *      type="sonata_type_collection",
     *      options={
     *          "by_reference"=false,
     *          "required"=false
     *      },
     *      fieldDescriptionOptions={
     *          "edit" = "inline",
     *          "inline" = "table"
     *      }
     * )
     */
    protected $documents;

How to instruct the form builder, to skip the documents, we should not load it at all? Any advice would be helpful.