Broadband customers using the internet at peak times get half the speeds they're promised, Ofcom has stated.
According to a report by the watchdog, households on a standard broadband deal offering speeds of up to 17Mbps often receive speeds of just 9.8Mbps in the evening.
As a result, they might struggle to complete day-to-day activities such as playing online games or streaming movies.
Ofcom said: "The speeds offered by providers are not consistent throughout the day and can fall significantly during busy periods due to people going online at the same time."
Figures from the regulator showed that the average UK broadband download speed has gone up in the last year, rising by 7.3Mbps to 36.2Mbps in the year to November 2016.
However, average download speeds drop to 34.6Mbps during the 8-10pm peak period.
Ofcom noted that speeds for cable customers generally compare favourably with other superfast fibre packages.
Nevertheless, it pointed out that a "significant minority" of Virgin Media's customers experience "severe slowdowns at peak times".
"For example, nine per cent of customers using its up to 50Mbps product and six per cent of customers using its up to 100Mbps product experienced average speeds of 10Mbps or less during peak periods," the watchdog said.
Despite this, Virgin Media's up to 200Mbps cable package achieved the fastest download speeds of all those compared by Ofcom.
The package averaged at 173.1Mbps over a 24-hour period and at 149.5Mbps during the 8-10pm peak period.