Cecilus at Mach 1000 looking at light moving at Mach 874,030 moving 100x slower
Btw, you still haven't proved why he would go SL, he doesn't have any reason to since its just a skeleton and a human present.
Amazingly, due the blitzing nature of the speed amp and the unknown timeframe for Rimuru's transformation (meaning it will be equalized to their base speed) will probably mean that Reinhard kills him before he transforms regardless. There is just no winning for Rimuru here
I doubt he will teleport 9000 kilometres away when he doesn't even have that much range, him transforming into SL is unlikely and Ainz is over a kilometres away and pretty helpless here