With clamav-openrc 20210506-1 I get a long pause at boot with it added to the default runlevel while the signatures are loaded, this takes several seconds on my system. Modifying /etc/init.d/clamd by adding the -b / --background arg:
start-stop-daemon --background --start --quiet \
fixes the hang. However in some use cases this may be undesirable if you really wanted clamd up before proceeding with the boot I expect, so perhaps it could be an option in /etc/conf.d/clamd that could be chosen. There might be a better way to avoid the hang that I don't know of. If you want to use clamdscan then you need clamd to run, and if you don't and just use clamscan the signatures are loaded every time.