Focus Punch describes the Speed Priority as a -3, but fails to account for the change in generations. According to Bulbapedia:
Focus Punch is a decreased priority move. The user of Focus Punch will charge up during the beginning of the turn it is executed, then attack at the end, unless it is hit by another Pokémon's attack before launching its own, in which case it will lose its focus and be unable to attack that turn. The user's focus will not be broken if its substitute is attacked, or if the opponent uses a status move (even if it deals damage, such as through the use of Pain Split).
Its charging phase has a priority of +8."
Focus Punch's charging phase has a priority of +6. If the user is hit before the attack, it will say, "<Pokémon> used Focus Punch!" before it says the "<Pokémon> lost its focus and couldn't move!"
It should at least mention the priority change somewhere on the move's page.
Bulbapedia also talks about priority moves as well, Serebii mentions there's a range from +5 to -7, but according to Bulbapedia its +5 to -5. So I don't know who to believe.