And, just for good measure, what are the CPU specs on the devices?
Dev tested on Asus PG258Q but wasn't able to reproduce the issue. Both Chrome and Opera are working the same and aren't laggy.
The MBP has a "2.9 GHz Intel Core i7".
The Windows PC has an i7 4820K at 4.5 GHz.
I wouldn't expect someone used to a 60 Hz display to be able to detect the lag. You need a 500+ Hz mouse so the cursor is smooth and clicks are instant, and the desktop should actually be running at 240 fps behind Opera so that you can see the contrasting difference in framerate. You can detect the delay when clicking the + icon to open a new tab. Also, clicking the Telegram icon slides in the window from the side bar at a low framerate.
The built-in pages like Settings, Extensions, and Personal News have horrendous scroll lag - they barely scroll at 30 fps. You can clearly see the difference in scrolling smoothness compared to Edge / Safari. I'm not sure if this viewport lag is related to the application UI or a separate Blink performance issue. https://testufo.com/ is able to render at 240 fps in the viewport.