Hibernation works (or doesn't in most cases) depending on your machine.
When hibernation is enabled, the system creates a file on the root of the C: drive called hiber.sys This file should be equal in size to the amount of RAM the system has. So, for example, if you have 4 GB of RAM on your system, the hiber.sys file will be of equal (sometimes greater) value.
That being said. When you wake your system from hibernate, you are accessing and opening up a 4GB file which contains instructions from the system as to it's state before hibernating.
In a laptop environment, where a battery is involved, the state differs. Laptops generally go into power saving mode before going into hibernate. Your network or multimedia may be shut off in order to conserve battery life (depends on power settings). Your processor might slow down (dimishing the voltage through the system) in order to conserve battery life. This will be true, even if the power cord is plugged in. The amount of time before this occurs will differ however depending on how you have configured the device.
All those factors have a direct effect on the hibernation or coming out of state of your machine. Furthermore, laptops generally have a 5400 RPM hard drive. 75% of the read/write performance of a regular 3.5" 7200 RPM drive. This means, that it takes longer for a laptop (generally speaking) to come out of hibernate. It takes longer for the system to read and process all of the information stored in the hiber.sys file.
Continue to factor in, most laptops have a single hard drive. That hard drive also has a pagefile.sys file. This can be set to a larger number, but is generally also the size of the quantity of RAM in the machine. So, in this example another 4 GB file. This is your "swap space" for moving data between RAM and system and allowing to artificially inflate the quantity of memory a system can theoretically store.
Once a system comes out of hibernate, it begins to re-initialize the powered down peripherals. This takes time. It than continues to load the state of the system.
I could keep on going. But I hope this helps you nuderstand why the system seems "sluggish" upon waking.
One last consideration to take note of... Nework connections. When re-awaking from hibernate, the network takes a few moments to re-initialize. In the meantime, any open network connections, etc. will generally cause explorer, for lack of a better term, bug out. Increasing the perception of sluggishness. The timeout for explorer to accept that the network connected drives, peripherals, etc. are no longer available is about 1 minute.
