  The design of a complex instrument such as Einstein Telescope (ET) is based
on a target sensitivity derived from an elaborate case for scientific
exploration. At the same time it incorporates many trade-off decisions to
maximise the scientific value by balancing the performance of the various
subsystems against the cost of the installation and operation. In this paper we
discuss the impact of a long signal recycling cavity (SRC) on the quantum noise
performance. We show the reduction in sensitivity due to a long SRC for an ET
high-frequency interferometer, provide details on possible compensations
schemes and suggest a reduction of the SRC length. We also recall details of
the trade-off between the length and optical losses for filter cavities, and
show the strict requirements for an ET low-frequency interferometer. Finally,
we present an alternative filter cavity design for an ET low-frequency
interferometer making use of a coupled cavity, and discuss the advantages of
the design in this context.
