By definition, nothing is stored in live cache against navigation attributes. So if you load a selection containing navigational attribute or drill down by navigational attribute, in theory this should be much faster vis a vis reading from live cache (not withstanding the computing theory of memory resident processing). This statement (of mine) is contra positive to SAP warnings on performance when using navigational attributes. There are a dozen notes on this to scare users.
There is not definitive answer I guess... at least empirically. It depends eventually on the query size. When query is large, it is hard to blame navigational attributes alone. It can be transaction time (SQL>LC routine > SQL) in the live cache as well. sometime reading a simple relational table (if indexed) can be much faster than reading non-relational (live cache) tables.
I am not a techspert and authority. This response is based on personal painful experiences.
Thanks
Borat