So yeah we have Relativistic speed Undertale because light dodging. This seems a bit strange to have none of them as FTL, since the feats was from a pretty weak monster and that we got things much faster than that before.
The mace is as tall as the one Knight Knight is using too!
So, I'll try to calc directly how fast certain attacks are compared to Knight Knight's bullets. Before we start, yes I know I'm using in-game seconds. However, Frisk's speed varies fastly depending on the opponent, and here I'm only going to use characters who are far stronger than Knight Knight herself.
Also, the size and distance of the SOUL in battle is also inconsistent, however, Knight Knight uses her Good MorningStar in her battle, so it implies this battle takes place in a up-close situation, same as the 2 characters I'm going to calc here.
Well first of all, to determine the light bullet's speed. In a single Frame, the bullet moves about 3 pixels away from its initial position as we can see there.
Then, comparing this to others characters' speed, Undyne. At the start and at many points in her fight, she swings her spear fast enough accross the screen that it is undodgeable. Undyne manages to cover 101 pixels in a single frame. 101/3=33. 33 times faster than light. FTL+ Undyne. Scales to Undying, possibly Mettaton EX and Frisk.
Then Asgore. He occasionally quickly swings his trident side by side accross the screen. The entirety of his trident reaches 121 pixels in a single frame. 121/3=40. So Asgore is 40 times faster than light, FTL+ mah boi. Scales to Toriel, NEO and Sans, possibly Undying and Firsk.
As a matter of fact, given that most people below Knight Knight scales from Napstablook for their Relat+ speed, it is probable that all of those people would have Relat+ to FTL speed like Napstablook do until we find a way to properly calcs Blooky's feat.