What sort of load do the tombstones create on the cluster?

[Maxim Potekhin]

In view of my unpleasant discovery last week that deletions in Cassandra 
lead to a very real
and serious performance loss, I'm working on a strategy of moving forward.
If the tombstones do cause such problem, where should I be looking for 
performance bottlenecks?
Is it disk, CPU or something else? Thing is, I don't see anything 
outstanding in my Ganglia plots.

TIA,

Maxim