Online retailers were not prepared for the first big shopping day of the year, says Site Confidence.
According to the web-monitoring firm, average download speeds of the UK's top etailers slowed to more than 11 seconds on November 29, which is below the desired target of eight seconds.
Some websites even experienced short periods of downtime, with a small number crashing for brief periods. However, there were no prolonged website outages.
Site Confidence has raised concerns that the reduced download speeds could result in a number of lost sales on Monday December 6, which has been dubbed 'Mega Monday' and is expected to be the biggest online shopping day of the year.
The firm said web users typically switch to an alternative website after an average of four seconds, meaning the prospect of an 11 second wait could damage sales as websites try to capitalise on the Christmas rush.
"These findings are significant as they show many of our top e-retailers have not prepared their websites sufficiently to deal with the vastly increased web traffic," said Bob Dowson, director at Site Confidence.
"Customers will simply switch to a rival brand providing the same products, if a website is failing to operate quickly or is not responding.
Dowson added it is now too late for retailers to prevent crashes or sluggish download speeds.
"Rigorous load testing should have been carried out to ensure these websites had the capacity to handle increased web traffic and maximise sales."