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).
That means this issue will be continued onto the Note 20 as well. I've been a loyal Samsung customer since the Note 2, but I absolutely will not buy another Samsung product until I see this issue gone on some future device. This infuriates me to no end that such an amazing device is crippled like this....
so this is what happenned at my end. In my post, I had written how Asus AX88U was not working well with my Note 10 Plus (resetting/ clearing logs did not work for me at all). I went back to my R7800 routers, all was well. I gave up on Wi-Fi 6 routers until one day I came across tp-link archer ax11000 which was on $100 discount on Costco. I had never thought tp-link as an option but boy, am I glad to try out this router! Excellent performance, blazing fast speeds, got even lower ping times, no issues with Note 10 plus, latest iPad 12.9 (Wifi 6), my wifi-6 laptops or older devices on wifi 5. I have felt no need to re-boot router periodically as it works smoothly. The router has 8 antennas, so I took off the second R7800 (acting as AP) as the wifi signal from this device is very strong. However, considered a very light-weight tp-link ax1800 as "just in case you need it!" access point in the same bedroom that needed the R7800 as AP. The tp-link ax1800 is 2x2 (most apple devices are still 2x2, as compared to samsung phones that are 4x4) and works on 80 Mhz channel width. I did not waste money on ax3000 as sole difference is that it is 2x2 on 160 Mhz, however, in practical sense or in automatic settings, most routers work on 80 Mhz so why waste money on 160 Mhz when you are going to use 80 Mhz only!
I have had ZERO issues with older and latest devices and both Tp-Link ax11000 or ax1800!
I have a new ZenWiFi mini XD4 setup, and it runs like a scalded dog for every Apple device in my house, including ones that aren't WiFi 6. My Note10+ gets under 1 Mbps sometimes when my wife's iPhone 11 Pro is getting over 400 Mbps. I doubt this is an ASUS problem when only Samsung devices are affected.
I have a OnePlus 7 Pro which has the exact same hardware (Snapdragon 855) as the S10 and Note 10. It also has the same radios since the radios are part of the chip. But I have zero issues on WiFi and cell reception, while the Note 10 was horribly slow with constant dropped signals. I bought a Note 10 because it's an amazing device, only to be let down by how slow the WiFi and cell reception was. If you look on this very forum, you'll see literally hundreds of posts about this issue.
Luckily I noticed it right away and returned my Note 10 for a full refund and am sticking with my 7 Pro. If the other poster above who has the S20 is having the same issues, that means the Note 20 will also. They either dumbed down the radios or cheaped out on the antennas. Either way, these devices are failing at their number one job of being phones first!