Mondrian query execution time (Saiku stuff?)

Hi forum users,
my cube is very very simple,
it has one measure (count on ID field of fact table) and one dimension with 2 level, Department and Office, both reference to the table OrganizationUnit.
The table OrganizationUnit has 35000 rows, with 3 Department values (Dept1, Dept2, Dept3).
Dept1 has 2 Office (Office1, Office2), Dept2 has 1 Office (Office3), Dept3 has 34997 Office (Office4, .....).
How it is possible that when I analyze the cube with Saiku and drag Office to filter section it takes a life and return:
OlapException: Mondrian Error:Query timeout of 300 seconds reached
(300 is the mondrian.rolap.queryTimeout property value in my mondrian.properties)
What should I investigate?
There is some mondrian setting?
Thanks all.