Hey there, Nikos Lagios,
Welcome to Microsoft Community.
To be honest, Performance improvements might vary depending on your device specifications and individual brower habits. Enabling the performance mode of the system does not provide a significant improvement for using Edge or Microsoft 365.
When you use your laptop while plugged in, the system runs in performance mode by default. When your laptop is not connected to power, it runs in power-saving mode by default.
However, some laptop brands and models, such as Asus, have built-in programs that can optimize software performance, depending on whether your current laptop brand has this feature.
Please let us know if anything is unclear.
Have a nice day and stay safe!
Kind regards,
Olivia | Microsoft Community Support Specialist