You are effectively doing a multi get. Getting more than one row at a time is normally faster, but there will be a drop off point where the improvements slow down. Run some tests. 

Also consider that each row you requests creates RF number of commands spread around the thread pools for the row. If one client req requests 100's or 1000's then this can delay other client requests. 

Cheers
 
-----------------
Aaron Morton
Freelance Cassandra Consultant
New Zealand

@aaronmorton

On 25/04/2013, at 8:29 PM, Thierry Templier <ttemplier@restlet.com> wrote:

Hello,

I wonder what are the performances of WHERE ... IN queries especially when the number of elements in the IN grows?

Thanks very much for your help!
Thierry