This is almost always due to large un-optimized images. Just as on a webpage, images need to be compressed so that they download quickly. If you are uploading PNG files (ending in .png) try using JPEGs (.jpg).
Check the resolution of your images and if they are over 1000 pixels in height or width, try scaling them down.
Most image editing software will let you pick a quality/compression setting for JPEGs, try setting it to around 70%.
Check the size of all the files you are using on the page, if they are over 2MB all together, the page may load visibly slowly.