DBVERPERFWARN1, Performance warning: Database aaaa is running in compatibility mode which degrades performance. Run MUPIP REORG UPGRADE for best overall performance.
Run Time Warning: This is a warning that the database is currently in compatibility (downgrade) mode. This mode causes all modified GDS blocks to be reformatted (to the downgraded database format) before they are flushed to the database file on disk. This is a very large performance hit.
Action: As the message indicates, run MUPIP REORG UPGRADE as soon as possible to move away from compatibility mode. This command can be run without taking the database offline. Once that completes successfully, the database is fully upgraded and there is no reformatting overhead anymore while flushing modified blocks to disk.