This document provides some performances analysis of running the end user portal in Pega 8.5.1 

Cosmos React has 5 times less transferred resources (Javascript and CSS) than Cosmos Rules and as such is twice faster in parsing the JS and 6 times faster rendering the UI. The home page DOM size is also 5 times smaller than Cosmos Rules.

By using less Javascript and CSS, having a smaller payload from the server, and a more lightweight DOM rendering, the Cosmos React implementation is 2 to 3 times faster on most of the use cases.