Replication 1 thread, very slow? (no replies)

i have about 400 connections in my master database, they keep inserting the records to the master.
my slave keep out of sync, seems can't catch up with my master.
i am not sure whether it's because of there is only 1 thread running sql statement in the slave.
for my slave server, both of the cpu and memory usage is not so high.
is there anyone met this problem before?
thanks.