It could be that something is running on your computer that's taking up nearly all of its processing power, meaning that there's not enough power to efficiently load web pages. The most common cause of that is security software (such as an antivirus program) that keeps trying to update itself but gets stuck in a loop. Some web browsers can also run loads of processes simultaneously, resulting in everything slowing down.
Launch Task Manager. If the Processes tab isn't already selected, click on it. The CPU column heading should be in single figures (e.g. 5%) or not much above it (e.g. 12%). The Memory column heading shouldn't be much above 50%. If either of those figures looks suspiciously high, click on the relevant column heading to place the biggest figures for that column at the top of the list. That will show what the dodgy process is.
(To launch Task Manager, right-click on the taskbar at the foot of your screen and select the option to do so. Alternatively, use the search function at the bottom left of your screen, typing 'Task' and then clicking on 'Task Manager' in the results above the search box).