Maybe you run more than one security software that interferes with each other. The resident protection of Spybot - Search & Destroy monitors the processes the whole time, so that nothing bad gets on your system - that can slow down your pc a little bit. If you do not want to have this feature you only need to disable the resident protection.

Please run Spybot-S&D and select "Spybot-S&D" --> "Immunize" in the navigation bar on the left. Please hit "Undo". Then open Spybot - Search & Destroy in the Advanced mode via the menu item Mode. Now select 'Tools' - 'Resident' from the navigation bar on the left. Please untick the checkboxes in front of the two tools.