Sorry Axel, but a few boxes with arrows don't really provide enough information to figure out what's going on.
We don't know the join types, conditions or cardinalities you use.
We don't know the level of aggregation.
We don't know the size and compression status of the tables, let alone the configuration of the system you're using.
If you want to know, where time is spend, using PlanViz is a pretty good start.
- Lars