Linux

20% is a LOT. That's probably because of the random shit that nobody ever asked for but windows is always doing in the background anyway. Building a search index, windows update (which consumes an insane amount of CPU for a completely unreasonable amount of time sometimes), other individual updater services (because there can't be one program that updates everything because every vendor does their own proprietary bullshit to handle updates), compressing and sending all you personal data to microsoft and of course the pre-installed McAffee (on trial license) that works hard to make your system less secure (that HP probably installed for you because apperently you haven't paid enough money for the computer, so you must pay with your patience and your privacy as well). Depending on the benchmark, the pathetic legacy file system windows uses might also play a role.