Not quite true.
The aircraft; whether the P-8 or the P-3 do not have to slow down to pin-point location of the Sub.
They both slow down, only to increase the on-station time by decreasing fuel consumption; this is called "loitering". Rather the P-8; because of its propulsion is able to reach the designated Patrol Area quicker than the P-3. So far as detection/pin-pointing the target/adversary is concerned; its the "Spook Gear" on board that does all the work, its hardly as if the Crew eye-ball the sub and shouts out "Sub Ahoy" while circling slowly!
![Mrgreen :D :D](/styles/default/xenforo/smilies/mrgreen.gif)
Its a simply a
no-brainer that the P-8 is a generation more advanced than the P-3. That should help you to put things into perspective.