The top U.S. internet service providers saw their prime-time streaming speeds for Netflix Inc. improve during December, with Verizon Communications Inc. registering the largest month-over-month gain among the big ISPs.

Verizon's Fios retained its No. 1 spot for providing the fastest streaming experience for Netflix in the U.S., registering average streaming speeds of 4.74 Mbps in December, compared to 4.65 Mbps a month earlier.

Cox Communications Inc., which tied with Altice USA Inc.'s Optimum service at second place in November, also registered a significant increase in streaming speeds. Cox took the No. 2 spot in December, with average speeds of 4.67 Mbps, up from 4.59 Mbps a month earlier. Meanwhile, Optimum tied with Charter Communications Inc.'s Spectrum service for the No. 3 spot, with each provider registering average speeds of 4.64 Mbps.

Nearly all of the 13 major U.S. internet providers tracked by Netflix showed a month-over-month improvement in average speeds.

Globally, Switzerland remained No. 1 for providing the fastest streaming speeds in a country-by-country comparison. The country averaged 4.80 Mbps, up from 4.62 Mbps a month earlier. Romania and Iceland appeared next in the speed ranking, with 4.76 Mbps and 4.66 Mbps, respectively.

The U.S., meanwhile, saw a month-over-month increase in average steaming speeds to 4.46 Mbps, up from 4.41 Mbps, but fell one spot among measured countries, ending December at No. 8.

Bolivia registered the largest month-over-month gain globally with average speeds of 3.20 Mbps up from 2.88 Mbps in November. Guatemala and Greece also registered gains in speed, to 2.98 Mbps and 3.97 Mbps, respectively.

Netflix's speed index measures prime-time video speeds for content streamed on Netflix's platform. It does not measure overall performance for other services or data that may travel across the network.