Magento 2 – Custom Product Attribute in Product Collection

So let's go trough this line by line

$_productCollection = Mage::getResourceModel('catalog/product_collection')
    ->addAttributeToFilter('popular_walldecal', array('eq' => 1))
    ->setPageSize(8);

Perfect, no need to change this. Just make sure you have the flat product data enabled under System > Configuration > Use Flat Catalog Product and the attribute popular_walldecal settings Use in product listing set to yes

$_collectionSize = $_productCollection->count();

Loads the whole collection, I suggest using getSize() instead of count since the first doesn't load the collection.

foreach ($_productCollection as $product):
    $_product = Mage::getModel("catalog/product")->load($product->getID());
endforeach;

Now here's the real issue. The collection just executes one query over one table (catalog_flat_product_[id]) but loading the product model in the loop executes a whole bunch of queries per iteration. It's better to add the required data to the collection.

$_productCollection = Mage::getResourceModel('catalog/product_collection')
   ->addAttributeToFilter('popular_walldecal', array('eq' => 1))
   ->addAttributeToSelect(Mage::getSingleton('catalog/config')->getProductAttributes())
   ->setPageSize(8);

$_productCollection->addFinalPrice()
   ->addUrlRewrite();

Mage::getSingleton('catalog/product_visibility')->addVisibleInCatalogFilterToCollection(_productCollection);
Mage::getSingleton('catalog/product_status')->addVisibleFilterToCollection($_productCollection);

This will

• filter by popular_walldecal
• add all the attributes in the catalog_flat_product_[id] table
• set the page size to 8
• add the final price and url rewrite data for the product url
• filter the collection by visibility in catalog
• filter the collection by status enabled

And now you're good to go!

You could try something like this. I believe the below method is quite efficient.

$attrData = array(
    'attribute_code_here'=> 'Default Value Here',
);
$storeId = 0;
$productIds = Mage::getModel('catalog/product')->getCollection()->getAllIds();
Mage::getModel("catalog/product_action")->updateAttributes(
    $productIds, 
    $attrData, 
    $storeId
);