Occuring only 2 days ago, my laptop has been experiencing a lot of lag, more specifically, directed to gaming. Though web browsing seems fine, still a bit slower than usual, the lag that I experience in gaming makes it near to impossible, such that I will lose connection or be kicked for 'timed out.'
Some things that might be helpful to know:
I hadn't been on my laptop for 3 days or so before getting on it 2 days ago.
I let my laptop 'AFK grind' for roughly 3-5 hours during the days prior to my 3 day absence from using my laptop.
Some devices don't register drops in speed as easily as others. That is quite common. Try this site for your tests, it gives much more accurate results than others I have tried. You can also set it to run multiple to give you an idea of the changes. I would also check it at different times of a day. If the major fluctation continues, then I would contact your ISP. Either they have an issue in your area (and don't let them say they don't as often they just don't know) or the modem could be in need of replacing. They should be able to resolve it on the call (unless, again, the modem is the issue). If it is their modem, then be sure to insist they replace it.