After I boot up, the very first time I open Window's Explorer and click on a folder I want to open, there's a 15 second delay before it actually opens the selected folder.

Thereafter, any time I click on a folder, it opens immediately. It's only the first use of Explorer after a fresh reboot where I see this 15 second delay.

It's almost as if the program is trying to get an initial "grasp" of all the folder data on my drive. But once it does, then it's fine.

Does anyone know what's causing this delay ?

How long do you wait before accessing a folder after the desktop appears after login? The typical computer today has numerous startup processes that run that may take several minutes to complete depending on the processing power and installed memory of the computer. The hard drive activity light will usually be running furiously during this time. In this case the computer is simply busy.

Real-time antiviral software or other utilities that hook file system calls can also introduce delays as entire folders may be scanned as they are accessed.

Is the delay behavior present if the computer is booted into safe mode?