The trusted client definition allows you to disable chained program and virus scan when a client source IP address or hostname match the search pattern. On regex found, SquidClamav falls back 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 not be bored by chained program and virus scan.