[Shinja]

First optimization is your loop, it's calling GetVehiclePoolSize at every iteration.
And a little (mistake or it's mean't to be like that?), it will respawn also used vehicles which will be annoying to players.

PHP Code:

for(new i, j=GetVehiclePoolSize(); i <= j; i++) 
    { 
        if(GetVehicleDriver(i) == INVALID_PLAYER_ID) SetVehicleToRespawn(i); 
    }