It's not the volume of distracting websites. It's the of seeing them before you're ready to make a decision about them.
Here's what happens: You open your browser to check documentation for work. Your bookmarks bar shows Reddit, Twitter, news sites. Now you're thinking about 6 different contexts instead of your original task.
That context switch costs an average of 23 minutes of focused work time. One "quick reference check" just derailed your morning.
The most productive people I've studied don't have better willpower. They control they see distracting options...