Orkney, Shetland and Argyll and Bute have come bottom of a table ranking local authority areas by broadband speeds, according to Which?
The consumer body analysed speed checker data in 32 council areas.
West Dunbartonshire, Inverclyde and Dundee have the fastest broadband speeds, the research suggested.
The Scottish government said delivering high-speed broadband in rural areas was more challenging but it was making good progress.
Highlands and Islands Enterprise (HIE) which leads the £410m Digital Scotland project, also pointed out that many customers had not yet taken up the highest speed connections available in their area.
Which? said its analysis suggested typical customers in Orkney had a speed of about three megabits per second (Mbps) while in Shetland it was 6.7Mbps and in Argyll and Bute 7Mbps.
Other areas with slow speeds were:
- Moray with 7.1Mbps
- Highlands -8.9Mbps
- Borders - 9.3Mbps
- Aberdeenshire and Perth and Kinross - 10.1Mbps
- Western Isles - 11.5Mbps.
Edinburgh was said to have a typical speed of 17.1Mbps and Glasgow 16.2.
Which? said broadband users in some of the areas could find it hard to carry out online banking or to use streaming services like Netflix or BBC iPlayer due to slow internet.
Alex Neill, from Which?, said: "It's incredibly frustrating that so many Scots are still struggling to get a good broadband connection when so many of us rely heavily on the internet to carry out important everyday tasks.
"The Scottish government must now press ahead with plans to provide 100% of the homes in Scotland with a decent broadband connection and make sure that no-one is at a disadvantage because of where they live."
Stuart Robertson, of HIE, said the Scottish government's Digital Scotland superfast broadband project had "dramatically changed" access to fibre broadband, with a new network bringing superfast to more than 250 towns and villages for the first time.
He said: "While some parts of these areas will experience slower speeds, the independent website thinkbroadband shows, for example, the mean download speed in Orkney and Shetland is 35Mbps. And in Moray, 86% of homes and businesses can access 30Mbps and above.
"There are still gaps in the hardest to reach areas, and the Scottish government has committed a further £600m in the Reaching 100% programme, in a bid to deliver a minimum of 30Mbps download to all Scottish homes."
'Best in Europe'
Paul Wheelhouse, minister for energy, connectivity and the islands, said the government's ambition was to make rural Scotland "one of the best connected places anywhere in Europe".
He said: "This report highlights what we already know - that it is more challenging to deliver and upgrade broadband infrastructure in rural areas.
"Our Reaching 100% (R100) programme is a plan to extend superfast broadband access to every home and business in the country in Scotland, and we have focussed procurement for R100 in rural areas, where it is needed most.
"A total of 93% of homes and businesses across Scotland already have access to superfast broadband, and we are investing £600m to go further."