load average

we have an unix system which has
load average normally about 20.

but while i am running a particular unix batch which performs heavy
operations on filesystem and database average load
reduces to 15.
how can we explain this situation?

while running that batch idle cpu time is about %60-65
while not running idle cpu time is about %70-80

how can i check swap status to determine if there is
a bottleneck or not?