Goku and Pikkon Cross the Afterlife[]
This is one amazing speed feat that base Goku and Pikkon are able to do. This is before the 7 year time skip between the Cell games and the Buu saga. They fly from Grand Kai's Planet, down to Hell.
Distance[]
Now why is this amazing? Because it is established that the afterlife is easily at least the size of the Universe.
And Grand Kai's planet is above heaven in every depiction and map, meaning they crossed down to heaven, across heaven, and down to Hell.
To conservatively low ball we will will use 1/2 the diameter of the Observable Universe, or 46.6 billion light years.
Time frame[]
As for time frame, no specific time frame is given, but it was a short time as Cell and Frieza were an immediate threat that they arrived to take care of before it got out of control, and came back from the battle in a relatively short time frame as well. We will low ball and use 10 min flight time to reach Hell.
Speed[]
SO 4.4087e+26m(distance traveled) / 600s(estimated time frame) = 7.34e23m/s
Speed = 7.34e23m/s = 2.45 Quadrillion c
Would be higher with transformations and other increases.
*It should be noted that the observable universe is a highly conservative estimate for distance. The true size of the universe is far larger to an unknown degree. The fact the actual distance is far larger, and the time frame is likely lower, makes this a severe low ball.