How can we load product image gallery instantly without the delay in magento?

Here's a simple solution that will cause the product's base image to be displayed while the JS is loading.

Create the following file in your theme: {theme_dir}/Magento_Catalog/templates/product/view/gallery.phtml containing:

<div class="gallery-placeholder _block-content-loading" data-gallery-role="gallery-placeholder">
    <div data-role="loader" class="loading-mask">
        <div class="loader">

            <img src="<?php echo $block->getGalleryImages()->getFirstItem()->getData('medium_image_url') ?>" style="max-width: 100%; max-height: 100%">

            <img src="<?php /* @escapeNotVerified */ echo $block->getViewFileUrl('images/loader-1.gif'); ?>"
                 alt="<?php /* @escapeNotVerified */ echo __('Loading...') ?>">
        </div>
    </div>
</div>
<!--Fix for jumping content. Loader must be the same size as gallery.-->
<script>
    var config = {
            "width": <?php /* @escapeNotVerified */ echo $block->getImageAttribute('product_page_image_medium', 'width'); ?>,
            "thumbheight": <?php /* @escapeNotVerified */ echo $block->getImageAttribute('product_page_image_small', 'height')
                        ?: $block->getImageAttribute('product_page_image_small', 'width'); ?>,
            "navtype": "<?php /* @escapeNotVerified */  echo $block->getVar("gallery/navtype"); ?>",
            "height": <?php /* @escapeNotVerified */ echo $block->getImageAttribute('product_page_image_medium', 'height'); ?>
        },
        thumbBarHeight = 0,
        loader = document.querySelectorAll('[data-gallery-role="gallery-placeholder"] [data-role="loader"]')[0];

    if (config.navtype === 'horizontal') {
        thumbBarHeight = config.thumbheight;
    }

    loader.style.paddingBottom = ( config.height / config.width * 100) + "%";
</script>
<script type="text/x-magento-init">
    {
        "[data-gallery-role=gallery-placeholder]": {
            "mage/gallery/gallery": {
                "mixins":["magnifier/magnify"],
                "magnifierOpts": <?php /* @escapeNotVerified */ echo $block->getMagnifier(); ?>,
                "data": <?php /* @escapeNotVerified */ echo $block->getGalleryImagesJson(); ?>,
                "options": {
                    "nav": "<?php /* @escapeNotVerified */ echo $block->getVar("gallery/nav"); ?>",
                    <?php if (($block->getVar("gallery/loop"))): ?>
                        "loop": <?php /* @escapeNotVerified */ echo $block->getVar("gallery/loop"); ?>,
                    <?php endif; ?>
                    <?php if (($block->getVar("gallery/keyboard"))): ?>
                        "keyboard": <?php /* @escapeNotVerified */ echo $block->getVar("gallery/keyboard"); ?>,
                    <?php endif; ?>
                    <?php if (($block->getVar("gallery/arrows"))): ?>
                        "arrows": <?php /* @escapeNotVerified */ echo $block->getVar("gallery/arrows"); ?>,
                    <?php endif; ?>
                    <?php if (($block->getVar("gallery/allowfullscreen"))): ?>
                        "allowfullscreen": <?php /* @escapeNotVerified */ echo $block->getVar("gallery/allowfullscreen"); ?>,
                    <?php endif; ?>
                    <?php if (($block->getVar("gallery/caption"))): ?>
                        "showCaption": <?php /* @escapeNotVerified */ echo $block->getVar("gallery/caption"); ?>,
                    <?php endif; ?>
                    "width": "<?php /* @escapeNotVerified */ echo $block->getImageAttribute('product_page_image_medium', 'width'); ?>",
                    "thumbwidth": "<?php /* @escapeNotVerified */ echo $block->getImageAttribute('product_page_image_small', 'width'); ?>",
                    <?php if ($block->getImageAttribute('product_page_image_small', 'height') || $block->getImageAttribute('product_page_image_small', 'width')): ?>
                        "thumbheight": <?php /* @escapeNotVerified */ echo $block->getImageAttribute('product_page_image_small', 'height')
                        ?: $block->getImageAttribute('product_page_image_small', 'width'); ?>,
                    <?php endif; ?>
                    <?php if ($block->getImageAttribute('product_page_image_medium', 'height') || $block->getImageAttribute('product_page_image_medium', 'width')): ?>
                        "height": <?php /* @escapeNotVerified */ echo $block->getImageAttribute('product_page_image_medium', 'height')
                        ?: $block->getImageAttribute('product_page_image_medium', 'width'); ?>,
                    <?php endif; ?>
                    <?php if ($block->getVar("gallery/transition/duration")): ?>
                        "transitionduration": <?php /* @escapeNotVerified */  echo $block->getVar("gallery/transition/duration"); ?>,
                    <?php endif; ?>
                    "transition": "<?php /* @escapeNotVerified */  echo $block->getVar("gallery/transition/effect"); ?>",
                    <?php if (($block->getVar("gallery/navarrows"))): ?>
                        "navarrows": <?php /* @escapeNotVerified */  echo $block->getVar("gallery/navarrows"); ?>,
                    <?php endif; ?>
                    "navtype": "<?php /* @escapeNotVerified */  echo $block->getVar("gallery/navtype"); ?>",
                    "navdir": "<?php /* @escapeNotVerified */  echo $block->getVar("gallery/navdir"); ?>"
                },
                "fullscreen": {
                    "nav": "<?php /* @escapeNotVerified */  echo $block->getVar("gallery/fullscreen/nav"); ?>",
                    <?php if ($block->getVar("gallery/fullscreen/loop")): ?>
                        "loop": <?php /* @escapeNotVerified */  echo $block->getVar("gallery/fullscreen/loop"); ?>,
                    <?php endif; ?>
                    "navdir": "<?php /* @escapeNotVerified */  echo $block->getVar("gallery/fullscreen/navdir"); ?>",
                    <?php if ($block->getVar("gallery/transition/navarrows")): ?>
                        "navarrows": <?php /* @escapeNotVerified */  echo $block->getVar("gallery/fullscreen/navarrows"); ?>,
                    <?php endif; ?>
                    "navtype": "<?php /* @escapeNotVerified */  echo $block->getVar("gallery/fullscreen/navtype"); ?>",
                    <?php if ($block->getVar("gallery/fullscreen/arrows")): ?>
                        "arrows": <?php /* @escapeNotVerified */  echo $block->getVar("gallery/fullscreen/arrows"); ?>,
                    <?php endif; ?>
                    <?php if ($block->getVar("gallery/fullscreen/caption")): ?>
                        "showCaption": <?php /* @escapeNotVerified */  echo $block->getVar("gallery/fullscreen/caption"); ?>,
                    <?php endif; ?>
                    <?php if ($block->getVar("gallery/fullscreen/transition/duration")): ?>
                        "transitionduration": <?php /* @escapeNotVerified */  echo $block->getVar("gallery/fullscreen/transition/duration"); ?>,
                    <?php endif; ?>
                    "transition": "<?php /* @escapeNotVerified */  echo $block->getVar("gallery/fullscreen/transition/effect"); ?>"
                },
                "breakpoints": <?php /* @escapeNotVerified */ echo $block->getBreakpoints(); ?>
            }
        }
    }
</script>

Hope this helps.

Aaron Allen's answer was great and i used it to figure out how to do it for my custom setup.

All i had to do was add the image with some inline css and add padding-bottom: 100% to the loading-mask.

...
<div class="gallery-placeholder _block-content-loading" data-gallery-role="gallery-placeholder">
    <div data-role="loader" class="loading-mask" style="padding-bottom:100%;">
        <div class="loader">
            <img src="<?php echo $_product->getData('img_url'); ?>" style="max-width: 100%; max-height: 100%; display: block;">
            <img src="<?php /* @escapeNotVerified */ echo $block->getViewFileUrl('images/loader-1.gif'); ?>"
                 alt="<?php /* @escapeNotVerified */ echo __('Loading...') ?>">
        </div>
    </div>
</div>
...

Its quite similar to the answer above but i wanted to add my 2 cents.