If you ever have an odd but specific problem with the Internet, it can be worth trying to same activity with a different Internet connection.
For example, if downloading from one particular site is very slow, or E-Mail suddenly becomes unreliable, the cause may not be with your machine. It could be caused by bad routing between your Internet provider and the service you are trying to access.
Unfortunately, the approach of trying a different Internet connection is not always possible. This is where a VPN comes in.
VPN stands for 'Virtual Private Network' and allows you to route your Internet traffic through a different provider. If the same fault disappears when your machine is connected to a VPN, then the source of the problem is very likely down to a fault with your Internet Service Provider (ISP).
VPN services are run by companies that normally charge a fee for access. In general, an Internet connection will work more slowly when running through a VPN because the traffic has to be routed through more systems.
The quality and speed of VPN services varies and many have restrictions on what the connection can be used for. Many do not allow P2P downloads, for example.
One service I have found very useful whilst diagnosing problems for clients is called 'Best UK VPN'. Its completely free, does not require registration and is easy to set-up. I haven't measured the speed whilst connected but it certainly works adequately for testing purposes. Indeed, websites appear to load as quickly as when connected without the VPN.