Barclays estimated Netflix is on pace to add 27.4 million global subscribers this year — an "incredible" rate of subscriber growth the firm said is nearly as many as HBO has added in the U.S. across the last 40 years.

Netflix has nearly matched last year's total subscribers added in three fewer months, Barclays noted, putting the online streaming service on track to set a new record this year. Not including Cinemax, HBO had around 30 million U.S. subscribers as of 2014, according to Barclays, a little more than 40 years after its start. HBO was founded in 1972 but first gained national distribution via satellite in 1975.

The firm's comparison gives a better understanding of how quickly Netflix is expanding both in the U.S. and abroad.