Nuclear submarines do not manage 10 times body length per second speed. But they do it in a sustained manner. If you average the speed of the octopi over the time it takes to recharge its head, you would find it is not doing so well either.
10 times body length per second is impressive on its own for under water bodies. I suspect the reason is the shrinking of the head as water is ejected out of the nozzle as it moves. In underwater craft, the vehicle has to displace the fluid around it, make room for itself and then occupy that space as it moves forward. Water is incompressible for all practical purposes. Water is a very heavy fluid, 1000 times the density of air ( 1 Kg/m^3 for air, 1000 Kg/m^3 for water). It takes lots of power to set that much of water in motion to move it out of the way. But octopi have an unusual mode of propulsion, its head will fill with water slowly and then when it moves its head is shrinking, it does not have to move that much water out of the way as it moves forward. That probably accounts for the relative speed.
Again, it is impressive for an octopi with tiny brains capable of just predicting football match winners. They could not have solved the Navier-Stokes equations and figured this out.