I'm going to get started on this early. Again, I'm going to use Tao Ren. and I am going to make one thing perfectly fucking clear before I do.
Eternal Sailor Moon is not moving at any speed within 6 orders of magnitude of the speed of light. To explain this, it's time for maths, boys and girls.
Mass is not constant. You may think it is, but it's not. The only reason that it seems that way is because nothing you can do can affect it in any meaningful way (other than dieting, which isn't what I'm talking about). No, in fact your mass increases the faster you travel. This happens because space and time is a whole lot more complicated than most people can fathom. You might be wondering why that doesn't affect you day to day. The reason is because you're not travelling fast enough. How fast do you have to travel? Well, let me show you the equation:
M = m0/sqrt(1 - (v^2/c^2))
This means that your relativistic mass is equal to your rest mass (which for Moon is estimated at 110 lbs, or 50 kg) divided by the square root of 1 minus your current speed (v) divided by the square of speed of light (c, roughly 300,000,000 meters per second).
At any given moment we (and Sailor Moon, assuming she's on earth) are travelling at 30,000 meters per second. That's only 0.0001% of the speed of light.
But to plug it into the formula:
M = 50/sqrt(1 - (9
00,000,000/90,000,000,0
00,000,000))
M = 50/sqrt(1 - (9/900,000,000))
By cancelling out the zeros, you can see that Moon's relativistic mass is equal to 50 kg divided by root 1 minus a fraction so small that it can be ignored. The square root of 1 is 1. Therefore her relativistic mass at earthlike speeds is equal to 50 kgs. However, Sailor Moon can travel far faster than the earth. What happens then? Well that's simple. Here's a picture.
What that shows is something called an asymptote, and what you can see happening as x approaches 0 (i.e. increasing towards infinity) is what happens to sailor moon's mass. As the figures for v and c get closer, the figure for v^2/c^2 gets closer to 1 (just like 3/4 is closer to 1 than 1/4) and thus the square root of 1 minus the fraction gets closer to 0. This means that the rest mass gets divided by a smaller number and thus increases (just like 1/0.5 is bigger than 1/5). You've probably noticed that they never actually reach 0 though. That's because dividing by zero is something you just
don't do. Maths fucks up when you divide by 0.
But that's OK, because it's impossible to make her speed equal to the speed of light because the mass as shown, mass rapidly increases the closer you get to the speed of light. When she's travelling at 90% of the speed of light, she weighs ~100 kg, at 97% that's doubled again to ~ 200 kg. At 99.9% of the speed of light she's at over 1,000 kg.
And here's where it becomes impossible for Sailor Moon to travel faster than the speed of light. There isn't enough energy in the universe for an object with mass to reach lightspeed. As the mass increases the energy required to accelerate it has to increase as well. As mass approaches infinity, so does the energy required to move a nigh infinite mass. According to the laws of thermodynamics, energy cannot be created or destroyed. It follows therefore that Sailor Moon cannot generate the infinite amount of energy required to travel at or above lightspeed. If the laws of Thermodynamics and relativity didn't apply in Sailor Moon's world, the universe would be fundamentally different, which it isn't beyond having magical girls in it.
I'm not, however Tastycles so I'm not even going to try to get into how fucked up spacetime gets when travelling that fast beyond "you get heavy enough to crush yourself with your own weight". I've probably made a few errors in my explanation too (this shit's complicated) but it's right in the essentials.
I am not accepting the speed of plot, so speed feats requiring travel between locations that cannot be easily found on a map of Japan (or if I'm feeling generous, an atlas) can fuck right off. Any objections?