To fly the Z warriors use their Ki; the reason they don't use their full speeds all the time is because it would drain more ki to fly at such high speeds, especially if such a thing isn't necessary, Goku dont use because he has the IT, in using Ki to fly, none of them has show light speed.
But in fights, The Z warriors move over sound speed since DB, and for short moments, they burst Ki to amplify his speed, strenght and Durability; a more ki, the burst of ki is higher and the Z move faster, For the high quantity of Ki Goku SSJ3 has, he sure could pass Over light speed, or be close to it, considering he is a guy who can dominate over 100G. Buu has the same speed that Goku SSJ3, so saying why Goku could not avoid Kid Buu attack is because they are fighting at high speed.
Piccolo explaining to Android 2 about Ki burst.
Naruto is a good show but the warriors from DB not Z(Yamcha, Tien, kid Goku, etc) could defeat all Naruto warriors .