This could be a problem to do with any extensions that you have as they can quite often slow down the speed of your browser. Another common issue are trojans that have made their way onto your PC. To fix this I advise you to download Malwarebytes which is a free anti-malware software which you can run and it will scan your computer for anything possibly malicious that could be on your device. Even if it doesn't fix your problem it is also a good thing to do.

If this doesn't work at all then try another browser and see if the problem persists.

Go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) and search for a download manager.