MySQL Cluster is pretty fast. The reason for this is, that it is completely memory based. Nowadays memory is still, in contrary to disk, limited to your systems. Thus, before installing a MySQL Cluster you have to calculate the amount of memory you need.
To say it in advance: You should consider to only use 64-bit Linux system with huge amount (4 - 64 GB) of RAM!
In release 5.1 MySQL Cluster became disk based. Now you have the possibility to swap out some data to disk. How much it is we will probably see a little further down...