Some customers of BT may experience delays in accessing some sites. A client of mine found accessing their site was painfully slow and made editing the site totally impractical. Tests from my end proved there was no fault with the server so I suggested it may be a DNS problem. Using tracert I discovered there was a slow response on the first hop. Changing the client's DNS to a reliable public DNS such that provided by Google solved the problem. You can learn how to change your DNS here : 

Windows 10
https://www.windowscentral.com/how-change-your-pcs-dns-settings-windows-10