WS2812B - Response time
Are you mixing up bits and bytes?

From the WS2812B datasheet:


Each frame is 24 bits.
Each bit takes 1.25 µs.
You have 512 frames.
At the end of the transmission you need a 50 µs reset code.


\$ t = 24 \times 1.25\mu \times 512 + 50\mu = 15.410 \ \mathrm {ms} \$

Refresh rate = \$ \frac {1}{15.41m} = 65 \$ updates per second.