With any character, strafing while walking forward has a higher top speed than walking or strafing by itself. The green character (I have no idea what their names are) has the fastest walking speed, at least early on.

Using his faster movement speed, in combination with the increase from strafe walking, it is possible to walk up rocks and slopes that you normally are not able to. This could lead to skips and sequence breaks if used properly. One thing that comes to mind is skipping needing to collect enough legos for a required hero mode build, especially early on.

How did you measure the speeds? I was under the impression that red was equal or slightly faster than green.

You're definitely right about the rocks and slopes bit. Here's an oob (actually quite easy) in the main hub that you can do with red, green, or blue. You can also use the recoil of brown to make it up, but that's a bit more tricky.

Curiously, I couldn't get this to work on pc, only on gamecube.

The first level of the game. I cleared it of enemies up to the boat and did several timed runs. The strafing was universally about 10-15% faster.

When you say strafe walking, were you using the strafe button on a controller? Or walking left/right while moving forwards? I was under the impression that you had the pc version.

Either way, cool find