[Twisted-Python] pb.Cacheable doc question

[Stephen Waterbury]

First, the "PB Copyable: Passing Complex Types" doc is
*great* and the examples are excellent -- my compliments to
all who contributed!

My question is about the pb.Cacheable section
(http://twistedmatrix.com/documents/current/core/howto/pb-copyable.html#auto9)
-- specifically the first sentence:  'Sometimes the object you
want to send to the remote process is big and slow. "big" means
it takes a lot of data (storage, network bandwidth, processing)
to represent its state.  "slow" means that state doesn't change
very frequently.'

I would think that the product of its size and its rate of change
is the applicable metric -- i.e.:  the bigger the object is *or*
the faster it changes (not the slower), the more applicable
Cacheable is, no?

Steve