Broadband speeds fall by more than a third during peak evening surfing times when most people are online.
Download rates are choked by up to 35% between 7pm and 9pm according to research conducted by Uswitch.
The study of more than 2m broadband speeds also showed that users in rural areas were much more vulnerable to drop-offs, partly because the infrastructure there is more susceptible to buckling under sudden increased demand.
The company sent users to its online speed test utility, and compared those postcodes which had more than 100 different results.
Internet users in Worcestershire were among the worst affected, with speeds dropping by 69% between peak and off-peak times.
In Weston-Super-Mare in Somerset the evening broadband speed (3.4 Mbps) was almost a third of that experienced in the morning (9.5 Mbps),
"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."
Recent campaigns by consumer groups have shown that the average broadband speed experienced by users is often far less than that advertised by providers.
According to Ofcom users regularly see speeds that are less than half of that advertised.
More than 50% of packages advertise speeds above 10Mbps, while the average speed received is just 6.8Mbps, according to the regulator.
Rules recently announced by the Committees of Advertising Practice (CAP) will stop providers advertising maximum broadband speeds unless 10% of customers actually receive that speed.
But campaigners including Which? have said that the new rules do not go far enough.
"Unlimited should mean unlimited at your normal broadband speed, but internet service providers will be allowed to slow down a supposedly 'unlimited' connection once a customer goes over a certain threshold," Which? said after the new rules were announced in September.
"Ofcom should step in where the advertising regulators have failed, and make sure consumers can't be misled about the broadband service they're paying for," it added.
There may be hope on the horizon, however. O2 recently announced a limited trial of its 'superfast' 4G mobile broadband service, which offers advertised speeds of up to 100 Mbps.