Aberdeen, Exeter and Hull are the worst for slow broadband.
Manchester and Birmingham rank first and second in the league table of UK towns and cities with the fastest internet speeds at the busiest times.
A study, based on more than three million consumer speed tests over the past year, found that Manchester enjoys average broadband speeds of 19.2Mbps during peak hours, while Birmingham experiences 18.8Mbps.
The research, by price comparison and switching service uSwitch, found that Aberdeen (7.9Mbps), Exeter (9.9 Mbps) and Hull (9.9 Mbps) experience the slowest peak time speeds.
The research also revealed that Exeter, Chester and Bath suffer the biggest drop offs between peak and off-peak hours, with speed more than 50% slower at 9pm than 5am, while London and Cardiff speeds drop by around a third, and Belfast by almost a fifth.
The findings coincide with other consumer research by uSwitch, which found that almost seven in 10 Brits says they have noticed their broadband speed is slower than usual at certain times of the day, with over half saying this happens usually between 8pm and 10pm.
About 45% of respondents said the slow broadband has affected their leisure time, 22% said they have lost an online auction, while 17% say they have missed out on online shopping deals during the sales.
"Overall, more than a quarter (26%) of consumers believe they have lost out financially due to their broadband slowing down to the tune of £159 each – so it’s hardly surprising that this has caused more than half (52%) to consider leaving their current broadband provider," the research said.
Ewan Taylor-Gibson, broadband expert at uSwitch.com, said: ""Given that most of us want to use our home broadband in the evening, it may be concerning to find out that the speed advertised when we sign up won’t necessarily be the speed we get at peak hours.
"People may also find that speedier fibre broadband is worth investing in – particularly for households with multiple connected devices. Just 44% of people think they can get fibre broadband – but in actual fact it’s now available to almost three quarters of the population."