Problems with your internet will often go way beyond your browser but if you are only having connectivity problems on one device or in one browser application, you might need to dive in and do some troubleshooting—it’s possible something is hogging your connection, maybe a browser extension or even a malware program.
Task Manager (search for it from the Windows taskbar) or Activity Monitor (search for it in macOS Spotlight) should give you some insight into where all your bandwidth is going, and should be of at least some help in tracking down where the problem lies. Quickly switching to another browser can help identify whether or not the browser itself is doing something to slow down the network.
This is one of those problems that you really need to throw everything at: disable add-ons, clear the cache, run a malware scan, and so on (but just try one at a time). If there’s something wrong with your internet connection in general, outside of your main browser app, we’ve got a guide to sorting that out here.