Steps to reproduce:
Open 20 tabs of some websites and try scrolling up and down for the bottom bar to show and hide, each time it hides or shows there will be a lag severity of which depends on count of opened tabs. Repeat in reader mode and try entering/exiting private mode.
Try again with 50 tabs (may be start page tabs too) and the lag will increase, plus some possibly independent unresponsiveness of UI elements.
Less impact to performance with intensive browsing activity (medium amounts of tabs opened at the same time)
Orion, OS version; hardware type:
App version: 188.8.131.52-beta (WebKit 86184.108.40.206.20)
System version: 15.1.0
Device type: iPhone 11
Native bounds: (828.0, 1792.0)