how to know imprints take effect?

Hi all, I am back, and I am using MonetDB Dec2016-SP5 with "enable-optimize" to do graph computing. I remember that imprints are suitable for all numeric types, however, after querying on the bigint column for many times, I found there is no imprints. What I want to achieve is to "make" MonetDB respond in milliseconds, although I know that in MonetDB it is not necessary to create index like imprints. So, on a table of 15,000,000 rows, can I expect to get response in milliseconds? and whether imprints will help with this? Any comments is appreciated! Best regards, Meng