Is Codeigniter 3 php 7 ready?

Be sure that the new version is faster.
The difference at the speed depends only on your code and how it behaves in the 2 versions (this includes and the framework code as well of course)!

* Wait for the Final version before taking any actions.
And I would recommend even to wait a few Fix versions to be pushed before you upgrade your server to the new PHP..
Usually the final new versions are not stable enough and soon after their release there are important to critical fixes added..

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