two quick conceptual questions RE: rsync (and rsyncing snapshots)

[user]

Hello,

I have started using rsync somewhat extensively and had two questions
regarding its operation.

First, how does rsync respond to, and perform, when the source filesystem
is under very heavy change ?  If I have a filesystem that I want to rsync
up to a backup server, but that filesystem is _very busy_ with the
creation, destruction and changing of files, how well does rsync perform,
and how much does it interfere with the performance of the underlying
filesystem that it is sending up to the backup server ?

Related: it occurs to me that perhaps it would be better to snapshot the
filesystem, mount the snapshot, and then rsync the snapshot.  On the other
hand, the filesystem is continuously altering the snapshot as files are
destroyed or changed ... so perhaps this does not gain anything.  Is
rsyncing a snapshot of a busy filesystem always, ever or never easier than
rsyncing the busy filesystem itself ?

Finally, am I correct that there are _only two_ rsync comparison methods -
the default checksum method, and the --size-only method ?  Am I correct
that rsync _always_ looks at the timestamp first, and then applies either
checksum or size comparison ONLY IF the timestamps are different ?

Thanks a lot.