Take a look at some of the amavis documentation about performance here:
zcat /usr/share/doc/amavisd-new/README.performance.gz | less
Also alot of the problems I have with mail queue getting stuck are to do with permissions so I do this:
adduser clamav amavis
and vice versa
adduser amavis clamav
You'd think this would be the default setup but apparently not always so.
Hope this helps