"If exponential growth is so fast, why did my two pennies only turn into four?"

It's been in the UK news for some time now.

In Mexico (where I live) we had the first Delta cases a couple of months ago. It is now the main variant and comprises 90% of sequenced samples (although Mexico doesn't sequence enough samples).

https://outbreak.info/location-reports?loc=MEX

Assuming this is actually a genuine question:

Here are the per country ratios of different VOC/VOI: https://covariants.org/per-country

It took a while until it became dominant in India, and then a similar pattern repeats in each new country.

In india.

Since most countries limit travel, it'll take longer to get around

You're being downvoted, but that's actually a good question. An explanation that fits the fact is antigenic drift. A variant that's different but not necessarily that much worse will outcompete another variant in which the population has built up significant immunity. The difference in fitness doesn't need to be that large.

If the virus truly had a massively larger R0 value, as is asserted in TFA, then you'd expect it to catch fire extremely quickly as soon as it starts showing up, and everywhere it is found. Ron Fouchier (see TWiV 777 as cited elsethread) certainly believes that these huge R0 values are not plausible, and that antigenic drift fits the observable facts better.

Its Wiki page shows where it has been since then; https://en.wikipedia.org/wiki/SARS-CoV-2_Delta_variant#Histo...

It's all over, and was all over the news early 2021 already. Not sure what you are talking about

(Or which kind of media you consume to come to that conclusion)

Covid (original version) took several months to spread around the world without something public health measures to slow it down. The Delta variant took about the same time despite the fact that significant portions have a degree of immunity from the vaccines or previous exposure, and much less international travel.