MY ISP says I should be getting 35 Mbps internet on my current plan but when i run a speed test i only get about 15 Mbps.
If i turn on a VPN (nothing else has changed), i then get the desired 34/35 Mbps speeds they say. is this down to my router or my ISP?
I have tried the obvious, defaulting to factory settings and reconnecting, changing the router position etc. i have changed my DNS settings to see if that has an affect. but the only thing that has an affect is turning a VPN on
If you find that connecting your VPN increases bandwidth, the most common situation is your ISP's preferred peering link to the target service was congested. ISPs have links to other parts of the internet, and they aren't evenly utilized. When you use a VPN, you are redirecting your traffic around the congestion.
A popular example is Netflix circa 2016. Tens of thousands of users were trying to watch Netflix and ISPs did not have enough high bandwidth links towards Netflix's data centers. To many people, this looked like ISPs were rate limitting Netflix, when in reality it was simply that all the bandwidth for the preferred route to Netflix was in use by customers. Telecommunications companies were slow to adapt to the rise in popularity of the service. In this situation, people using VPNs could exit their ISP's network on another uncongested link, arrive at their VPN provider's data center, then go to Netflix via a different route available to their VPN provider.
The rules that choose which link to use are typically sensitive to distance and monetary cost. Congestion isn't a common metric, so it is very typical to see this sort of issue.
TL;DR, Try a different speed test. Ookla typically has servers inside your ISP's network. You should always get the advertised speed on a Ookla test.
