[supertrope]

Lots of websites are bloated with megabytes of poorly optimized Javascript such as advertising network tracking and commenting systems. Animated images tend to drink CPU time as well.

iPhones still have significantly faster CPUs, especially single threaded.

The end result is that even though web browsing is typically categorized as a basic computing activity (compared with media production, 3D gaming, or simulations) it actually requires regular CPU upgrades to keep up with more complex/bloated websites. There are obviously other factors like whether or not the system is memory pressured, SSD or not, use of GPU acceleration, and if the Internet connection is adequate.