Well I'm here to clear up some things about Thor's speed.
Well I was looking back at some past comics and I've discovered some things about Thor's speed.
In Journey into the Mystery number 110 where Thor runs so fast as to be invisible to the human eye. In order for one to be invisible to the human eye they must be running faster than the speed of light.
An impressive, but lesser feat is in Journey into the Mystery 112 where Thor runs the entire length of New York in a "heartbeat", and in Thor V2 Thor fights Thialfi at super speeds.
His speed has been potrayed a bit inconsistently.
Flying, he can get anywhere as fast as he needs to, and can teleport dimensions. On the ground, it's debatable.
Part of it is speed isn't a heavily used part of his power. He doesn't depend on it to accomplish a goal. He's a warrior who would rather hammer his way through an object or opponent, rather than use speed for victory.
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"I'm not smart so much as I am not dumb." - Harlan Ellison
I've seen Thor spin his hammer at such high speeds that the molecules around the hammer catch fire, but then, later on, he can't chase down a car on foot. Very inconsistent.
If I HAD to say how fast Thor is I'd say he's somewhere around Spiderman's level in speed. Since Thor is as strong as the Hulk, he can probably run a few hundred miles per hour if he gets a good start.
Lightning moves much faster that the speed of sound yet lightning is still visible to the human eye. In fact while in a vacuum Lightning moves at the speed of light, much faster that sound.
I'm not sure, but aren't some planes and or jets able to move at the speed of sound?
Its a possibility but think about it first the lava is oing straight down and then in one panel where we see Thor in multiple places at once and all of a sudden there is a ditch right there.
Spiderman could do the same feat IMO. Thor is simply moving fast enough that he's a hard-to-see blur. Spiderman is said to be much faster than the human eye and he's got feats out the ass that prove it.
Unless we're using metaphors and such like "Faster than lightening" as fact, Thor isn't even super-sonic. It doesn't make him less of a great character, but throughout my Thor collection, his speed feats are nearly nonexistent.