Did you do a firmware upgrade? During the transition you will see that message, then it will go back to wrtite-back. In the EQL GUI, select the member, then the "Controllers" tab. You should see both controllers with same firmware. Under "Memory Cache" you will see the status of "write-back" or "write-through"
If this is correct, most common causes of performance issues in ESXi is not being in compliance with the best practices of EQL with ESXi.