I am completely surprised to find that my Galaxy Note 9 bought in India has started to download the stable Android 10 update some time back. It's a 1.9GB sized update. Happy to see a One UI 2.0 based update for my Note 9 just post new year.
This is a stable update for all as I was not part of the Samsung beta program for Galaxy Note 9 Android 10 update.
You may feel that the device has slowed down post successful update on first boot. But that is not the issue as device actually needs time to complete the update fully post the first boot after applying the update. I will suggest that better leave the phone for few hours like 1-2 hours minimum to settle. Post that the Note 9 will be again as smooth as it was with Android 9.
Recd the Jan 2020 security patch update and post this update the device is now quite smooth in terms of day to day UI experience. Still not as smooth as it was with Android 9 One UI 1.5 but definitely way better than original Android 10 update UI performance. I am quite sure that with Feb or March updates the Note 9 UI will be very smooth.