Hull, with an average of 12.4Mbps, is the UK's slowest city, according to comparison site uSwitch, which conducted the tests.
London and Edinburgh had average speeds of 22.4Mbps and 21Mbps respectively.
These speeds are likely to be low not because of lack of availability but because people have chosen not to take up faster services.
Government figures suggest superfast broadband - 24Mbps or higher -  is now available to more than 90% of homes and businesses, with funding pledged to bring that total to 95% by 2017.
According to BT, its fibre services are available to 24 million home, but only 22% (5.5 million) have connected to them.
Hull is the only UK city not served by BT's Openreach, which controls the telecoms network. Hull's independent supplier, KCOM, disputed the findings.
"We're rolling out ultrafast fibre to the home across Hull and East Yorkshire, which means speeds of 250Mbps are available to consumers right now," said a KCOM spokesman.
The company says it is connecting "a new customer to fibre every 30 minutes".
But uSwitch's Ewan Taylor-Gibson said: "Actual speed tests taken by broadband users would suggest this hasn't reached enough homes yet to make an impact on the average."
Three in 10 tests logged speeds of less than 5Mbps with just one in 10 registering speeds of above 50Mbps, according to the data.
"We should be asking what more can be done to encourage the adoption of superfast broadband now it's so widely available," he said.
"The UK's towns and cities should be leading the charge when it comes to broadband speeds, yet just 22 cities have broadband users with average speeds of more than 24Mbps."
Co-founder of broadband news site Thinkbroadband Sebastien Lahtinen said: "Broadband speed tests are a great way to measure real world speeds experienced by users - but they also serve as a measure of take-up, not just availability."
"Whilst early adopters and digital natives are often craving for faster and faster services, not everyone wants to stream multiple HD streams live whilst playing online games.
Those who just use the Internet for the occasional catch-up TV programme, email and Facebook, broadband speed is not really a priority, especially when the fastest services can cost considerably more."
Tests on 42 towns and cities across the UK suggest almost half have average broadband speeds below 24Mbps.