I am currently using Avira (and have used many different brands in the past).
Most of this depends on the computer that the antivirus software is being run on, what the computer is being used for, and how much left over resources that the system has.
On my system, Avira uses about 48 megabytes of memory and runs a complete system scan once a week at 2am. The scan takes about 12 minutes to read my two SSDs and check 700 gigabytes of files. When I launch an application, it also does a scan of the file and any library files that are loaded.
My computer is fast, though, and I only run SSD drives. On my slowest laptop computer (Intel i3 CPU, 5400 rpm disk), with a normal hard drive, launching a normal application takes about 15% longer. It all seems to be basically fine.
Two of my sisters have had problems with computer speed when running antivirus programs, but that was using eight year old computers with almost no free disk space with torrent systems running on them
As N3rdR4ge said, putting antivirus on an already messed up computer won't fix it...