You need a flag variable for when you want the speed to be increased or decreased. So for example:
Every X seconds
---->Slow = 0
--------->Set Bullet.Speed to Bullet.Speed + X
---->Slow =1
--------->Set Bullet.Speed to Bullet.Speed - X
Bullet.Speed = 300 (You might want to make it <= because its not guaranteed you'll land on 300 exactly depending how much you are adding)
---->Set Slow = 0
Then just set Slow to 1 when you want time to slow down.