When hibernation is enabled on a computer, a hiberfil.sys file is created in the root of the system drive. The hiberfil.sys file is sized to hold the all the data that could be in memory.

Performance on a system with more than 4 gigabytes of memory that has hibernation enabled is very very poor, causing Microsoft to disable hibernation on these systems.

If you must have hibernation support on your x64 system, decrease the installed RAM to 4 GB or less.