At times, the program can be disk intensive, but that is during file scanning, which is what can cause the performance hits, and delays in opening certain files, or running certain programs.

How much a performance hit degrades your overall system performance depends on the hardware installed. Such things as the speed of your processor, and the amount of RAM, and the type, and speed of your hard drive can also decide such things, not to mention the amount of fragmentation on your hard drive.

On most, if not all modern PCs, the performance hits that Avast! gives will not be significant enough to hamper your system, the main exception to that being when it finds a threat, and has to lock that threat down.

As a note, when running an active scan that you initiate (quick scan, complete scan, custom scan etc) it’s recommended that you do not use your PC during the scan, both for performance, and security reasons.