Cluster slow down when one sql node is down (no replies)

Hi,

I just started to try MySQL Cluster. One test I'm doing is to take down one sql node while populating data into Cluster. I noticed the insert slowed down dramatically while one sql node is offline. Once that sql node is back online, then everything is back to normal.
Can you explain the slowness? Or where can I look into it?

Thanks,

Jacky