World of Warcraft is a large and ever-growing game. In order to help save you hard drive space, the World of Warcraft Optimisation process will periodically "clean up" your install folder by removing or replacing redundant game data. Please avoid closing the World of Warcraft Launcher while the optimization process is in progress.
The process itself requires an additional 2-3 GB of hard drive space for "room to work", but once it's complete your WoW install folder will typically be much smaller. The optimisation does not involve defragmentation, and is safe for solid state drives.
Should you encounter any issues with a patch that included this optimisation, be advised that the issue is likely being caused by a variable that is unrelated to the optimisation process itself. Please see our regular patch troubleshooting guide for assistance.