Some store owners have often enquired why wishlist button doesn’t load at the same time as most elements of page (e.g. Add to Cart button)?
The simple answer to this is that Add to cart button is core component of Shopify, while Wishlist button is a component of 3rd party App.
Our App is triggered by Shopify only after the store page has fully loaded. In layman terms, as soon as the loading spinner of the browser stops, the App gets triggered and the Wishlist button gets displayed. The time gap between stopping of loading spinner and display of wishlist button is always less than 0.5 seconds. In fact, on average it is between 0.1-0.2 seconds.
The only exception to this happens once when a new user directly lands on product/collection page. In that case, it takes little more than 1 seconds. For the remaining part of his browsing session and during subsequent visits, the App Cache takes over and due to that the Wishlist button is displayed within 0.5 seconds.
For performance reasons, Shopify doesn’t trigger the App before page load finishes.
Demonstration on a client store