One of the important things to understand about how browsers read your CSS selectors, is that they read them from right to left.

This means the first thing that is read in the rule .header > *:nth-child(3) is :nth-child(3)

Looking closely at the performance issues documented in bug 64709 - a big cause of the lag is this selector. Removing it from ui.less improves performance to the same standard as the desktop site.

Version: unspecified
Severity: normal