When you start or restart your computer, there are a lot of things that take place that make your computer rather unresponsive. Windows and Bitdefender Antivirus plus and possibly others are checking in with their masters to see if there are any updates to be downloaded and processed. Plus there are a number of things that Windows has to do to get itself set up and running fully. If you open Task Manager, you will notice a number down at the bottom of the window that tells how much the CPU (Central Processing Unit) is doing. Until you see that number settle down to something around 5%, it is still doing its thing. If you will just be a bit patient until it finishes setting up, your system will be more responsive.
If your computer has not been started for several days, it is critical you wait till these updates are completed. The most dangerous infections are the newest and you want that protection.