Also tell us what you are trying to capture. If you are trying to capture the desktop with "Monitor Capture" make sure you disable Aero. Tick the checkbox in the Video settings saying "Disable aero at startup" and then restart OBS. If you are trying to capture a game, use Game capture and make sure Aero is ON.
Also one more thing to note: Use only one capture method in one scene. unchecking the source doesn't "kill" the source. It's still there slowing down the performance. So always own scene for Monitor, Window and Game Capture.