I bought a M1 MacBook Pro 2 weeks ago with 16 GB unified memory, it is running macOS 11.2.3 and Safari 14.0.3.
And webpage scrolling was choppy from Day 1. It happens on Apple's own websites. Or even on websites that do not have any ads.
Scrolling over text would be fine until the images in the page appear.
This issue exists in Chrome also but to a lesser extent.
The UI stuttering and sluggishness happens outside Safari too, like the Notification Center.
When connected to an external display, the stuttering would happen when moving windows.
I also reported a dozen other issues using the Feedback web page. https://www.apple.com/feedback/
After speaking to Support, I completely erased the drive and reinstalled macOS.
The issue exists even on a fresh install without any 3rd party extensions and apps.