Ahhh, okay, this makes more sense. didn't realize you hadn't played the game. Alright here, I'll provide some relevant links and explain the speed of light calc, the speed of the beam calc, and exactly how crazy this is.
At this point in this video you will see a battle with a foe called Knight Knight. Knight Knight attacks with sunlight, via a miniature sun. I've calced the light coming from this sun moving at a speed of twice the size of the SOUL that's dodging it per second. Without getting into my previous timescale calcs that determine how quickly the perception of Frisk must be to percieve light moving that slowly by determining the size of the soul and scaling the light to it, we can still directly correlate the speed of light calc to the speed of sans' beam, which appears instantaneous on at least this timescale.
Looking at this picture we can see that the beams cross the entire screen, and in the fight, they do so instantaneously, that is to say in a single frame, or 1/40th of a second. Since the size of the soul does not change between fights, we can take the size of the soul, multiply it by two, and measure across the length of the beam, then multiply by 40 to determine the bare minimum multiplicative factor of the speed of light the beam travels at. Assuming the beam is at least 10x the doubled size of the soul, likely closer to 15, but let's lowball it here, we're looking at about 400x the speed of light.
This is how we know that Gasterblasters are ridiculously faster than light.
Now, if we want to bring in my timescale calculations, which have been accepted in the past, we can determine exactly how fast the reaction speed of Frisk must be in order to percieve light at such a degree. These calculations assume the size of the soul is 5ft for simplicity's sake, however, the soul is likely smaller, as such these should be considered lowballed figures,
significantly.
If Undertale dodging took place in normal time the light would travel at 9993.08 kilometers per frame. The speed of light in an actual Undertale dodging segment is .0001016 kilometers per frame.
Meaning the timescale of an Undertale dodging stage is... 983578866.61 x actual speed. or 29507365998.3 'frames' per actual second.
As you can see, Frisk is capable of dodging back and forth in front of the light in the couple of seconds it takes to get to them via their perceptive timeframe, and Sans is capable of stealing the initiative from Frisk and dodging (effectually, no less) their attacks, even though their reaction speed is that high. As we all know, if your opponent can reflexively react to your dodging faster than you can react to them, you will be hit in any kind of combat, thus we can determine that sans must be able to react fast enough to dodge attacks from beings that percieve light that slowly. This is assuming my timescale calculations are accepted, of course. Regardless, the fact remains that the blasters are well beyond the speed of light.