Persuading people to pay $119 a year for Prime membership has been crucial to prompting people to shop more on Amazon. This is why it keeps adding Prime benefits, like streaming TV and discounts at Whole Foods — and, soon, more free one-day shipping.

“We think that will open up a lot of potential purchases and convenience to those customers,” Mr. Olsavsky said.

But membership growth has been slowing.

About two years ago, three-quarters of people who had free Prime trials would sign up for paid memberships, but now fewer than two-thirds do so, according to Consumer Intelligence Research Partners, which surveys users for brands and investors.

While customers without a Prime membership have annually spent about $600 on the site the last several years, the amount that Prime members spend has increased each year by about $100, up to an average of roughly $1,400 in 2018, according to the research firm.

“Non-Prime customers have little incentive to shop more — when they do, they just become Prime members,” said Michael Levin, a partner at the research firm.

Amazon has also reported slower growth internationally. In countries like Germany and Britain, there are fewer Prime subscribers and fewer products eligible for free, fast shipping with Prime, said John Blackledge, an analyst at the investment bank Cowen.

Amazon has also struggled in developing economies. This month, it said it would close its domestic Chinese online business after years of gaining little traction with consumers, and it has faced logistical and regulatory roadblocks in India, where it has invested heavily.