I don't know what the problem is, but if it's any comfort, mine has started doing the same thing since the last update. Mind you, yours does take considerably longer than mine (approx 5 minutes for a 200 Gb disk).
OnePlus 5 review
Alice Saey's mesmerising animation for Dutch singer Mark Lotterman
iPad Pro 10.5in (2017) review
Comment booster votre iPhone ?