The reason less “visually complex” websites are considered more beautiful is partly because low-complexity websites don’t require our eyes and brain to work as hard to decode, store, and process information. Watch this short video about how the eye sends information to the brain to understand what I mean.
Basically, your retina converts visual information from the real world into electrical impulses. Those impulses are then routed through the appropriate photoreceptor cells to transmit color and light information to the brain.
The more color and light variations on the page (i.e. the greater the visual complexity), the more work the eye has to do to send information to the brain.
Researcher Saul McLeod explains that