i have figured out that the vfs cache make us get a long standby time
If changed vfs cache back to default (100) you will have the normal continues usage
sooooo:
i'm thinking to make a script that when we lock the screen of n900 the vfs cache changed to 10 which will give us a long standby time while n900 is locked
and when we unlock the screen the vfs cache changes back to 100
which will give us a long continues usage time

super perfect idea
but not super perfect me to do that kind of scripts
i don't have a lot of info about how does N900 manages the screen lock and unlock