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The size when gzipped is only 3kb, and performance is as good as (or even faster than) React itself.
Pro Same API as react
Avoids having to learn an entire new paradigm, and makes it easy to switch between the two libraries later on.
Pro Lots of examples and libraries
They have a bunch of examples and addon libraries.
Con Difficult to learn
You need to be good in react js first to learn preact. Documentation is not rich. And some of the examples given on site are not working. Developers are not much active on stackoverflow as well.