Previously you had to use an ancient SMS based SIM app on your phone or use a dongle to authenticate, took over a minute usually.
A way for retailers to "bypass" 3DS is to use Klarna or similar (free in-app invoice that needs to be paid within 14 days). Even though it's usually quite simple to use my debit card, it's still more of a hassle than paying whenever I want within 14 days, so that's what I choose when I'm in a hurry.