@mdn,

I don't have an N9 so I cannot check. What's the (default) value of /proc/sys/vm/dirty_background_ratio?

Perhaps the N9, given its huge available memory, has a large percentage of memory available for the write-cache, so that when it actually has to write everything to "disk" the whole system halts for a while.

The N900 has ratio = 10%, which is the default in the kernel.