If you are running a full system scan using Norton, the process could take several hours depending on the number of files on your computer. During this time, some system resources are consumed by the antivirus, slowing down other programs and even the operating system itself. You can schedule when these scans occur in the software settings to avoid conflicts.
Norton performing slowly on your computer may not be entirely down to Norton 360 but may be caused by other tasks running in the background if you notice a slowdown when booting your PC. Windows Task Manager lists the processes running in the background, and we'd advise disabling unnecessary processes to boost performance.