heibernation

yes it is because of hibernation because when you shut down your PC there are several tasks that run at shut down like(it clears the page file and it clears ram also)... so when computer do not perform these tasks for a long time then they affect your computer and because of these all things your computer get slower...

hibernation will decrease the performance of your pc, as it is constantly updating the hibernation file to the current state that windows is in, so that when you press hibernate it will save your current state. this will munch through the cpu allocation and make your pc go slower. if it not a necessity to hibernate then dont do it.

Well leaving your computer on too long can cause performance issues because more and more items are being loaded into the RAM, causing your computer to start accessing the page swap. Hibernation writes all the data in the RAM to your hard disk. This can be bad if you are low on Disk Space, and its recommended you restart your computer if it gets too slow.