The trusted client definition allow you to disable chained program and virus scan when a client source ip address or hostname match the search pattern. On regex found SquidClamav fallback to Squid instantly. Of course you must have Squid authentication helper enabled.
For example:
192\.168\.1\.10 myhomepc 10\.18\.10\..*
will let clients with ip 192.168.1.10, with hostname myhomepc and any host under the 10.18.10.0/24 subnet to not be bored by chained program and virus scan.