There is the s
ame discussion on NF about this subject, and I'll just quote a certain person:
"
basically Kizaru's light's first problem is because of its portrayal.
1. it can be stationary -> so it isn't always lightspeed.
2. it can move at a very slow pace -> so again it isn't always lightspeed. We have seen this when he connected his upper and lower body.
3. Brook have seen kizaru's light before Kizaru could translocate from one location to another -> which basically points that Kizaru is far more slower than his own light.
4. Rayleigh literally stopped kizaru after his light has traveled but before kizaru got sent to that intended location, which proves point 3.
5. and more of the same shit that again will still affect the input of the databook.
basically Kizaru himself before he becomes light due to his traveling ability is slower than lightspeed.
Hence he is only lightspeed when traveling through the mirror.
"