Is Amazon Prime maxing out?

The e-commerce giant’s rewards club, which offers paying members two-day free shipping on millions of items and streaming video programs, grew just 8 percent over the past 12 months to 97 million U.S. subscribers, according to CIRP, the slowest pace since the research firm started tracking membership in 2012.

CIRP says Prime subscribers increased by 2 million in the most recent quarter, which included Amazon.com Inc.’s Prime Day sales promotion.

“Prime membership growth has clearly flattened,” said Josh Lowitz, partner and co-founder at CIRP. “Amazon Prime membership has now spread far beyond the early adopters who were presumably the most committed Amazon shoppers.”

Amazon couldn’t be reached outside office hours.

Walmart Inc. has tried to blunt the appeal of Prime membership over the past year by offering free two-day shipping on orders of $35 or more, a tactic that’s been copied by other retailers. That said, Prime members are still buying tons of stuff, spending an average of $1,400 a year on Amazon, compared with $600 for non-Prime customers, CIRP found.