When the carousel is activated, the following will load all variant ids
in the cache (cf. `_in_cache_without`):
```
<t t-set="variant_img" t-value="any(product.mapped('product_variant_ids.image_variant'))"/>
```

When accessing the `image` field at:
```
<div t-if="variant_img" class="..." itemprop="image" t-field="product.product_variant_id.image" t-options="..."/>
```
All variants in cache will have their image resized by the method
`_compute_images` on `product.product`. In case of a product with
hundreds of variants with images, this will take a major amount of time.

If we replace by `product[:1]`, the system uses another cache, and
doesn't fetch the image of all variants.

opw-1844783