Yum- disable database update on package install
Passing -C to yum will tell it to only use the metadata in the current cache. Note that this will cause the operation to fail if it needs metadata that is not currently in the cache, or if the metadata from the various repositories are not in sync.
"... Can that be set in a config file somewhere? Passing -C every time may get tedious. ..."
Update YUM config file and set metadata_expire variable to a higher value (default is 90 mins.) You can then use yum makecache command to manually update the metadata as and when required.