Performance Reading Cube data

From a level I'm executing a <b>layout A</b> (with just a dynamic column) which shows plan data. (It takes 2 seconds to read from the plan cube)

From the same level I'm executing a <b>layout B</b> (with just a dynamic column) which displays actual data (It takes 2 seconds to read from the actual cube).

When I create a <b>layout C</b> that contains the two dynamic columns the system takes 80 seconds. (2 seconds to read the plan cube and 76 seconds to read the actual cube)

<b>How can this be possible if I'm working from the same level and i'm displaying the same actual data as in Layout B</b>?

Thanks. points awarded to useful answers.

Alberto