S21 Ultra 16GB Ram is turning to S4/S5 after one UI 4.0 upgrade

[TejasB]

Folks I'm facing severe slowness issue especially while making / receiving calls on my S21 Ultra, 16 GB ram version. 

The issue cropped up immediately after I upgraded to One UI 4.0 

Need suggestions on how to fix the issue. 

Thanks in advance...

[cs-member13]

Dear Samsung member,

Greetings from Samsung customer support.

Thank you for sharing your concern. And please follow the easy troubleshooting steps which might help to solve the issue.
Check space availability:

Is there sufficient space available on your device?
Kindly check the space on your device:
Path: Settings > Device Care >Storage

To improve the device performance there should be sufficient space. Request you to keep at least a ''minimum 20% internal memory free''.

Kindly check RAM in your device:-
Path: Settings>Device care>Memory.

To improve the performance RAM of the device should be free. So, clear the RAM memory once a day.
Check app cache
Does the device hang while accessing any particular Application/Game etc?

Please follow the below steps
1- Clear app cache through Device care:

Step1: Navigate to the app's screen and tap on Settings.
Step2: Tap on Device care.
Step3: Tap on Storage.
Step4: Tap on CLEAN NOW.

Thank you for writing to Samsung.
Warm regard,
Samsung customer support

[TejasB]

I've done all of the above recommended multiple times, but it's all the same. One UI 4.0 is working **bleep** on my 16 RAM phone. Please tell me how I return to 3.0