Thats the one bad thing about bluray. It has big slowdown issues on non-sequential access. it's fine for a film where it just runs from start to finish, and for basic data storage it's great coz of the size. But a game has to jump back and forth, loading bits here and there.
Uncharted 2 is shown off amazingly on the format though by having multiple copies of the same data at various points on the disc so that it never has too far to look for a particular file. terribly inefficient from a programmer's point of view, but then again Sony do best at hardware. :)