I had the same problem a few months ago and this is how I fixed it. If you have a Smart Wi-Fi router like Linksys, make you sure you're using the 5 GHz band instead of the 2.4 GHz and also you may wanna play with channels and channel width, apply each one and then check the speed each time until you get the maximum speed. Once you're able to get around 150 mbps, make sure you remember that channel number and width in case you reset your router at a later time. Speedtest.net may be a good website to test your speed. Hope it helps.
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