Get this Doc as PDFTurn off power saving functions, if there are any (those reduce the CPU speed when the battery gets low).
Turn off CPU hungry services like WIFI, GPS, Bluetooth, etc. or apps with heavy CPU or audio usage. Running the device in flight mode for example gives you a lot more performance headroom.
To turn off an app or service, go to:
System Settings / Apps / <whatever app you want to close> / Force Stop
A fresh boot may help a lot in some situations, since it resets all background processes.
Also keep in mind that Android throttles the CPU when the device is getting too hot. Therefore in some cases, taking the device out of a cover (e.g. some hard cover or flip cover), can bring it back no normal temperature, and so to a higher CPU frequency.