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 Thialf at super speeds.
In Thor 354, Thor was toying with Hela using speed that according to her was beyond comprehension.
Thor on a few occasions has been described as being as quick as the very lightning he commands. The main stroke of lightning travels at about 0.1 c (30 million meters/second or 100 million feet/second) and the peak current lasts for tens of microseconds or so. After the peak, the current typically decays over tens or hundreds of microseconds. In otrher words Lightning is slightly slower than the speed of light, but in a vacuum their speeds are the same.
Once The High Evolutionary using Thor’s DNA, created a real Super Speedster in Zefra.