Best paragraph in the article:

  So, what caused all this extra work that slowed down  
  page loads? The majority of the work came from  
  JavaScript that the Brave browser injected into every  
  page to help its adblocker. Specifically, it was the  
  cosmetic filter — a component responsible for adjusting  
  page layouts to correct for breakages caused by  
  adblocking. Brave’s developers didn’t need to work on  
  the script to improve its performance; they needed to  
  delete it.