Logic-Level Optimization的解释不懂

逻辑级优化中的两个过程，结构化与展开化，手册上是这么写的，为什么展开化没有了中介结构和变量之后反而速度变化，只有两级逻辑结构，那如果逻辑复杂的话不也很慢吗?没有看很懂，请高手简单解释下，
Logic-Level Optimization
Logic-level optimization works on the GTECH netlist. It consists of the following two
processes:
• Structuring
This process adds intermediate variables and logic structure to a design, which can result
in reduced design area. Structuring is constraint based. It is best applied to noncritical
timing paths.
During structuring, Design Compiler searches for subfunctions that can be factored out
and evaluates these factors, based on the size of the factor and the number of times the
factor appears in the design. Design Compiler turns the subfunctions that most reduce
the logic into intermediate variables and factors them out of the design equations.
• Flattening
The goal of this process is to convert combinational logic paths of the design to a
two-level, sum-of-products representation. Flattening is carried out independently of
constraints. It is useful for speed optimization because it leads to just two levels of
combinational logic.
During flattening, Design Compiler removes all intermediate variables, and therefore all
its associated logic structure, from a design.

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