I don't know anyone in the US who is still on 3G and modern CPUs are not 4x slower than their desktop counterparts.
Also modern CPUs on mobile are faaaaaar slower than 4x the desktop one for the majority of the world.
It's still wild to me that the 2013 MacBook Pro that was my daily driver until recently is neck-and-neck on Geekbench with both my Pixel 5 (whose CPU is considered mid-range) and the old iPhone 7 that I use as a test device. It's decisively slower than every iPhone since version 8.
If we move ahead to modern desktops: it looks like iPhones have typically been only 20 - 25% slower than iPad Pros released in the same years, and this year's iPad Pro literally has a desktop processor in it (not even just a laptop processor, now that the iMac uses M1 too).
Based on that, in order for your claim to be true, the majority of the world outside the US would have to be using outdated or very low-end mobile devices and/or modern souped-up desktops that blow the average American's machine out of the water.
Some googling shows that a popular phone in India is the Redmi 8, which is pretty low-end even compared to the Pixel 5, and scores about half the 2013 MBP at multi-core and slightly above 25% at single-core. If the average owner of a phone like this also happened to own a modern (not 8-year-old) mid-range consumer laptop, I could see 4x being overly optimistic.
B) I didn't say they did, but not every product needs to be concerned with the rest of the world. Websites in English targeted at a US market and charging US Dollars for their products probably don't care much about the average mobile processor speed in India (anecdotal source: me).
I would guess that most SaaS businesses are primarily accessed on desktop computers during the work week, but I bet they're now collectively spending millions-if-not-billions of dollars in dev time to make their landing pages load faster on Indian mobile phones for users who are unlikely to ever visit or become customers.
(I pick on India because my site gets a lot of Indian traffic, but feel free to swap in the developing nation of your choice.)