Download speeds suddenly drop but only specific devices effected

DIIIIIII

Distinguished
May 14, 2014
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3 days ago i was casually using my pc when i noticed that things were loading much slower than than usual, naturally i just restarted my modem hoping it would fix the issue but it didn't. I then proceeded to test multiple devices only to find that some where getting the issue i was, whereas others weren't. None of the devices that were getting the slow speeds had any new software or updates installed, i made sure of this by going into windows updates and checking the most recently installed programs listed in windows.
At this point i took to the forums and found similar issues, however most people found that it was due to recently installed software, which wasnt the case for me. I also noticed that while testing my speeds on speedtest the results were consistently bad (0.14mb down 40mb up), however while testing through my isp's server on speedtest i found that my speeds were normal (70mb down 40 up). To add to that, i also tested my speeds over steam, only to find out that my normal download speeds hadn't changed. Games such as PUBG also had issues even loading into the main menu as my net was too slow. At this point i had no idea what to do, I'm almost certain it is on my end and not my ISP's as all the phones and a single PC weren't effected.

Any help at all would be much appreciated,
Cheers,
Dejan.