I've been thinking this over, and I've come up with some concrete figures for Geralt of Rivia from The Witcher franchise's speed. Let's take a look at a canon book quote:
"The witcher struck with the speed and force of a lightning bolt..."
- The Last Wish
Now lets look up just how fast that is. I typed in "Speed of lightning" and got the following as the first featured result:
"The return stroke (the current that cause the visible flash) moves upward at a ). In comparison, the sound of thunder travels at about 1100 ft per second or about 750 miles per hour."
So it's pretty clear from this evidence that Geralt can actually move his swords at a third of the speed of light. What makes this all the more impressive is that it was never shown to be his max and he was pretty casual throughout that fight. (It was with Tailes, who posed no threat to him for those of you who don't remember.) But that's not all. Take a look at this:
"‘Watch out, Geralt!’
He dodged again at the last minute. Brilliant lightning ploughed a furrow in the ground,
the air hissed with colour, murderously sharp fragments from broken windows."
- The Lady of the Lake
See how casual of a lightning timer Geralt is. For his perception, he had a whole minute to react to moving lightning. This shows that him being lightning speed in a bare minimum and he is in actuality well above this. Let's just say twice as fast.
How do you all feel that Geralt of Rivia is at least two thirds as fast as the speed of light?