I have the same problem (super slow, almost NO WiFi) on my Note9, first showing up with the last Android 10/One UI v. 2 update that was installed on March 1, 2020. It wasn't occurring before this Android 10 update. It appears to be intermittent, sometimes lasting 2-3 days, then working again, then reoccurring. I have 2 other Samsung Galaxy devices that do NOT show this problem on our router (an ASUS RT-AC3200). 1. Galaxy S7 Edge with Android 8 (Oreo); 2. Galaxy Tab S3 with Android 9 (Pie). NOTE: these 2 other Samsung devices are only operating on WiFi (and not connected to a mobile provider), in case Samsung technical support reads this. I also have many other devices on this router with no issues. I searched for Galaxy devices slow WiFi and found this problem was around in 2015. Maybe it was an old software bug that accidentally got back in Samsung's releases? I've had my Note9 since October 2018 and it did not have this problem until the most recent March 1, 2020 Android 10 update and security patch. The solution that fixed it today, for my Note9, was to do these 2 steps (steps that others have said didn't work, some say it works): 1. Forget the WiFi connection (Settings>Connections>Wi-Fi>Click on 3 dots upper right & choose "Advanced">Manage Networks>Click & hold on the "current network" so it is selected>Click "Delete" on the upper right") 2. Reconnect to the network We'll see how long this work-around will work, and if the problem comes back again. I think I tried this solution previously, but still had the problem come back. I read that others have tried this and it doesn't work. (My Speedtest App results for when the Note9 was working with fast Wifi on this router: Download 492Mbps, Upload 154Mbps. Speedtest results for when it's not working: Download 0.16Mbps, Upload 1.69Mbps. I have Speedtest results on this phone since Feb 2, 2020).
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