How do I measure the performance impact of antivirus applications?
AV-TEST state that their method is to use a large number of files &amp; record the time taken to download, then repeat using each antivirus &amp; see how much it slows down.


  Partly disappointing system loads  
  
  For the load test of the security packages, 839 files, representing a
  volume of 30.6 GB, were copied on a reference system, and many
  downloads were performed. The time needed for these tasks served as
  the reference value in the test. As soon as a protection solution was
  installed, it was required to complete the same tasks. The relevant
  times required are listed in the latest test chart. The results,
  however, could not be more diverse. The reference time was 246
  seconds. As already mentioned, the packages from Bitdefender and ESET,
  with 249 and 253 seconds, hardly put the brakes on the system. The
  other values from Symantec, Canimaan Software, Kaspersky and AVG are
  good up to the clocked 268 seconds, however they're not great. But
  after Sophos with 284 seconds, the values continue to increase
  significantly: from 292 to 466 for Avast. The nearly unbelievable
  value of 1387 seconds was measured for Dr. Web. That is more than 5.6
  times the reference value.


[the referenced chart]



In my own, less than scientific tests, I could feel the slowdown using Sophos or Avast, long before I read about them on the AV-TEST site.

Regarding system load, they appear to be less specific. One can only imagine they are looking at Activity Monitor, but there's no direct information, only the rather vague


  The two charts with the test results quickly reveal that the
  protection solutions from Bitdefender and ESET, despite 100% detection
  rate, cause little or no burden on the system. They require for their
  own performance a mere 1 to 2% of system resources. Most of the other
  solutions in fact require up to 10% system resources for security
  performance, in some cases even m ore.