Oz
Normal flame front is no where near the speed of sound. A quick calculation will show you that even if you don't believe the data produced by others. Based on a IO 520 engine with a 4" stroke and a 5.25" bore operating at 2400 RPM and set to fire at 22 degrees of crank angle BTDC so the fuel burn finishes around 14 degrees ATDC. the following can be calculated.
The fuel burn lasts for 36 degrees of crank rotation. At 2400 RPM the crank rotates 40 times / second and one rotation (360 degrees ) takes 0.025 seconds. This means the fuel burn takes 0.0025 seconds. With both plugs firing each flame front only has to travel about half the the piston/ bore diameter ie 2.625" or 0.06667 metres .Using only my I- phone here on the shores of Lake Superior I make that 26. 67 m/sec or 96 kilometers/hr average.
Remember also that even in the best of engines the fuel air mix is far from homogenous. Pinging starts randomly in places where the temp and mixture most favor auto ignition. The TEL delays any auto ignition event from getting properly started . If an event does get going properly no amount of TEL will stop it.
Cheers RA