There's many possible reasons for that:
- different workspace locations (local vs. network storage)
- massive slave/master communication for "Slave x" case, e.g. caused by
- lots of console output
- storing huge amounts of artifact data (will pass from slave via master to storage)
- different JVM settings for Slave x and Master x
Otherwise, you will need to monitor in detail.
Once you monitor in detail you might figure out that sending data from slave to master might be an issue.
Just check it out.