When my user goes on one of our category pages, for each product in that category, the photo, the name of the product, the price of that product and I imagine the SKU of that product are all loaded by Magento. How many additional pieces of information are loaded for each product? Is this adjustable? Could I go in to Magento's code somewhere and change it so it only loads, say, the name of the product, the image and the price for every product on my category page?
On category page Magento will load all those product attributes which are set to
Yes in Used in Product Listing. It also load the attributes which are used in Layered Navigation filter.
If you don't want an attribute to load on product listing page then you can change the above configurations from Admin.
To change configurations go to Stores-> Attributes -> Product. Edit that attribute you want to remove, now under Storefront Properties you will find this configuration to Use in Layered Navigation and Used in Product Listing