Usually the largest saving.
WHY IMAGES FIRST
They are typically the majority of transferred bytes.
WHAT TO DO
Size them to their displayed dimensions Compress appropriately Use modern efficient formats, with fallbacks Serve different sizes for different screens Defer anything below the visible area
WHAT NOT TO DO
Upload images straight from a camera or phone Scale a large image down with styling
WHY THAT SECOND POINT
The full file is still downloaded.
WHAT FORMAT TO USE
Modern compressed formats for photographs Vector formats for logos and icons
WHY VECTORS
They are small and scale to any size.
WHAT TO ALWAYS SET
Dimensions, so space is reserved.
WHAT TO DO ABOUT VIDEO
Never autoplay with sound Never load it before it is wanted Use a preview image that loads the player on interaction
WHY
Video is enormous, and most visitors never play it.
WHAT TO CHECK
The transferred size of each image, in the network panel.
WHAT TO AUTOMATE
Optimisation on upload, so it does not depend on anyone remembering.
