UK broadband speeds drop by an average of 35% from their off-peak highs when most people are online in the evening, according to a report.
The research, conducted by the comparison site uSwitch, was based on two million broadband speed tests.
The peak surfing times between 7pm and 9pm were the slowest to be online, the report said.
There were also huge regional variations between evening and early morning surfing times.
The report suggested the best time to be online was between 2am and 3am.
Users in Evesham, Worcestershire, fared worst, according to the survey, with a massive 69% drop-off between off-peak morning and evening surfing.
Those living in Weston-super-Mare did little better with speeds falling from an off-peak average of 9.5Mbps (megabits per second) to 3.4Mbps in the evening - a 64% drop.
The difference was often most noticeable in rural areas where even peak speeds were relatively slow. In Wadebridge, in Cornwall, speeds nearly halved from 4.1Mbps at off-peak times to 2.1Mbps at peak times.
"It really is surprising just how much broadband speeds fluctuate at different times of the day, with drop-offs of almost 70% in some areas of the UK," said uSwitch's technology expert Ernest Doku.
"Not many internet users enjoy the maximum headline broadband speeds offered by providers, and certainly not during the working week," he added.
Broadband speed is becoming more important as bandwidth-hungry services such as on-demand TV become more popular.
Telecoms regulator Ofcom recently revealed that British households download an average of 17 gigabytes of data every month over their home broadband connections.
That monthly data diet is equivalent to streaming 11 movies or 12 hours of BBC programmes via iPlayer.
Critics say consumers are being misled by internet service providers who continue to advertise their maximum broadband speeds, even though many users do not get them.
New rules from the Committee of Advertising Practice (CAP) say that from April next year providers will no longer be able to advertise maximum speeds for net packages unless 10% of customers receive them. They must also make it clear if a significant proportion of users wll not be able to get the headline speed.
Almost half of broadband users are now on packages with advertised speeds above 10Mbps but the average broadband speed is 6.8Mbps according to Ofcom.