Calc Keys To The Kingdom - Tom throws his spear

So the Mariner can throw his spear fast.

‘There will be time enough,’ said the Mariner with a well-gauged look out at the distant aerial battle, the smoke, and the nearing dragonfly. ‘Provided we do not stop to gossip. I’d best let that old wormsnake know we’re coming up.’

He lifted the harpoon above his head. Arthur heard its crackling paper noise and tensed for the toothache and joint pain that would strike when it flew. But as the shaft of light leaped from the Mariner’s hand to flash up the hillside, Arthur experienced no more than a passing twinge.

Within a second, the harpoon, moving too fast to see clearly, slapped back into the Mariner’s open hand. - Lord Sunday

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How far away is the Mariner from the hill and the wormsnake?

A second later, there was another deafening boom and the earth shook as the fireball hit the ground about half a mile away and smashed its way through a dozen or more tall hedges and garden beds, starting even more fires. - Lord Sunday

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It says within a second by for conviencences sake let's round it up to a second.
Half a mile = 804.672m

Timeframe = 1 second

804.67200m/1s = Mach 2.3459

Final results
Tom throws his harpoon = Mach 2.3459

He would likely scale to Grim Tuesdays feat and the Sunsprites, but consistancy is good. Lady Friday dodged his harpoon at the last moment, so this scales to her too.