RetroNX (a group consisting of @m4xw
) has just released a public beta for his overclocking and underclocking system module called sys-clk. You may have already heard about another tool named Freebird which has a similiar goal in mind however, unlike it, this one works on a per-title basis - meaning you can have different performance profiles for your game that get automatically loaded as soon as you launch them. Said profiles can also be edited on the fly and the changes will take effect immediately, plus there's also support for having separate docked/handheld configurations.
CPU, GPU an RAM clockspeeds can all be changed and some of them can be pushed even higher than the official limits for docked mode. Still, the homebrew does not allow you to set them to dangerously high levels to keep your system safe.
The system module is compatible with both Atmosphère and ReiNX, is completely open source and does not crash when putting the console to sleep if active. However, while a GUI is reportedly in the works, the only currently available way to manage your profiles is to manually add them to an INI file ( /config/sys-clk/config.ini ) on your microSD card using a text editor.
The developers also maintain a database with readily available presets for popular titles, which can be reached via a link on the official thread.
Keep in mind that the homebrew is still in beta and may contain bugs. Read the official thread and documentation for detailed info and installation instructions!
Official GBAtemp thread