PX4 logs detailed aircraft performance data once the system has been armed until it is disarmed. These flight logs can also be used to analyze performance issues.
Keeping flight logs is a legal requirement in some jurisdictions.
Downloading Logs from the Flight Controller
Logs can be downloaded using QGroundControl (v3.2 or later): Analyze View > Log Download.
Sharing the Log Files for Review by PX4 Developers
The topic Flight Log Analysis (PX4 Developer Guide) contains basic information about log file analysis (and alternative analysis tools).