Windows 8.1 and Windows Server 2012 R2 General Availability Update Rollup

This update rollup includes the following performance and reliability improvements:

• Push-button reset and boot experience: Stabilizes the boot experience. Fits and finishes changes in push-button reset (PBR) and refresh. (Client and Server)
• In-box apps: Provides reliability and stabilization fixes for inbox apps, including Camera, Photo, and Video Viewer. Stabilizes SkyDrive integration with inbox apps. (Client)
• Graphics and multimedia platform:  Improves graphics and multimedia reliability and performance, including stabilized visuals across power states. Improves 3D performance. Improves the handling of PlayTo. (Client)
• File server and clustering: Improves Enterprise Client Sync experience. Resolves reliability issues in continuous availability (CA) and backup scenarios involving failover clustering, Hyper-V, iSCSI and DedDup. (Server)
• Hyper-V: Provides general virtualization reliability fixes in Hyper-V, Network Virtualization, and Remote Desktop Virtualization for large deployments. (Server)

More information can be found here and here;