There is always CPU and RAM involved for every nice component you use. Just how much the penalty is depends completely on your hardware, index and type of query. Under heavy load it numbers will change.
Since we don't know your situation and it's hard to predict without benchmarks, you should really do the tests yourself. > Just wanted to know how efficient field collapsing is. And if there is a > performance penalty, how big is it likely to be? > > I'm interested in using field collapsing with faceting. > > Thanks.
