Since the iPhone 12 will be the first Apple smartphone with built-in 5G connectivity to be released this fall, prospective buyers naturally want to know which carriers offer the best 5G performance.

OpenSignal recently released a new report measuring the global 5G experience of 10 mobile operators.

On the plus side, all U.S. carriers' 5G networks offer significantly faster performance than 4G. The bad news is that 5G speeds vary widely from provider to provider, as does the availability of 5G signals.

As expected, Verizon's 5G network, which uses high-speed millimeter wave technology, had the highest average 5G download speed. Verizon achieved 506.1 Mbps, beating out Sprint (114.2 Mbps), AT&T (62.7 Mbps), and T-Mobile (just 47 Mbps).

This speed difference is not surprising. Verizon's initial 5G strategy is based on millimeter wave, which offers much faster speeds than LTE. Other carriers are initially using low-to-mid bandwidth spectrum, which offers more modest speed gains. [But here's a caveat: according to OpenSignal's findings, Verizon's 5G utilization is only 0.9%, compared to T-Mobile's nearly 20%. That's because T-Mobile's 5G network network network, which uses the 600 MHz band, reaches much farther than Verizon's short-range millimeter wave network.T-Mobile launched its nationwide 5G network in December, and the San Francisco Bay Area, Sacramento, Calif, and just announced further coverage expansions to Tampa and Orlando, Florida.

AT&T's network, operating in the 850 MHz spectrum band, also offers relatively good 5G coverage compared to Verizon, with an availability rating of 9.7%. Sprint's network operates in the 2.5 GHz band, also known as the mid-band frequency band.

The good news for T-Mobile is that now that it has acquired Sprint, it will soon be able to offer both better speeds and better coverage if it can combine the networks T-Mobile already has in Philadelphia and New York, Sprint's 5G coverage into their networks, and the networks in these two cities use low-band, mid-band, and mmWave technologies. This should mean faster speeds and wider availability, but has yet to be confirmed due to constraints at the shelters.

Verizon is not standing still either. The carrier says it will double its mmWave coverage from 30 to 60 cities by the end of the year. Verizon has also stated that it will offer mid-band 5G during the third quarter of this year. In addition, Verizon just turned on 5G upload speeds, which are about 30% faster than uploads over 4G LTE.

AT&T announced in April that its 5G network now covers more than 120 million people and 190 markets AT&T announced in April that its 5G network now covers more than 190 markets and 120 million people.

So which 5G network is best suited for the iPhone 12? Rumor has it that AT&T, Sprint, and T-Mobile would be the best carriers for these two devices at this time, as the regular iPhone 12 and iPhone 12 Max only support networks below 6 GHz.

The iPhone 12 Pro and iPhone 12 Pro Max are both expected to support mmWave and sub-6 GHz, so they should fly on Verizon's network. However, there is still a lot of time before October, when the iPhone 12 lineup is expected to be announced.

Therefore, carriers will have to continually improve the 5G experience to accommodate what the iPhone 12 and iPhone 12 Pro can offer.