From product testing I had read out that Bitdefender is not such a big performance hogs. I can 8.1 with Intel Core i7 confirm (by the end of 2013) only partially since installation on Windows. Yes, for some time the scanner runs with very low CPU load. Suddenly, however, the fan runs faster. I close all open programs. The task manager shows a processor load of about 30 percent for a single Bitdefender process (and in addition, there are still running other Bitdefenderprozesse). This puts the computer pretty lame.
After bad experiences with Kaspersky, McAfee and Norton I'll maybe go back to Antivir.