it's mentioned in bold right on the installation page http://docs.ajenti.org/man/install/general.html, and also in APT output after installing

Could it be that you forgot to use https://?

Please upgrade to 1.0 beta (see our website for instructions)

Where, in your opinion, the performance impact is worst? Startup time, response time or in any particular plugin?

Alrighty, that's it. Betas are 0.99.x builds, and release will be 1.0.0

Please let us know what browser are you using.

Fixed in the latest build

No, installing Ajenti will not change your existing configuration.