Why blake2b is the fastest despite it being 512 bits hashing algorithm? I am struggling to understand why blake2b is faster than blake2s, despite that blake2b is 512 bits hashing. I hope someone can clarify that for me. I read the official paper for blake2b, but I am not a very technical guy. I didn't understand much of it.
I am struggling to understand why blake2b is faster than blake2s, despite that blake2b is 512 bits hashing.

Comparing is not easy. They designed

BLAKE2b for 64-bit platforms including NEON enabled ARMs. That can produce between 1 and 64-byte digest

BLAKE2s for 8- to 32-bit platforms. That can produce between 1 and 32-byte digest


They are fast on their target

Each one is portable to any CPU, but can be up twice as fast when used
on the CPU size for which it is optimized; for example, on a Tegra 2 (32-bit ARMv7-based SoC) BLAKE2s is expected to be about twice as fast as BLAKE2b, whereas on an AMD A10-5800K (64-bit, Piledriver microarchitecture), BLAKE2b is expected to be more than 1.5 times as fast as BLAKE2s.

Even BLAKE2b does 12 rounds, and BLAKE2s does 10 rounds it still faster on their platform. And the CPU word size has a huge effect, too.
They modified the original design of BLAKE with the confidence it's security well tested. From the Third-Round Report of the SHA-3 Cryptographic Hash Algorithm Competition

Keccak received a significant amount of cryptanalysis, although not quite the depth of analysis applied to BLAKE, Grøstl, or Skein

Some of the modifications; BLAKE2b uses 24-bit rotations that use SSS3 and can perform two rotations on parallel. Originally BLAKE was using 25 (now 24) and 11 (now 63) bit rotations. In addition, the 63-bit rotation can be also fastened on some recent Intel CPUs.
AVX and AVX2 (Advanced Vector Extensions) can boost any code whenever applicable. BLAKE2b benefits form AVX/2 whenever available. More detail can be found at BLAKE and 256-bit advanced vector extensions paper.
As having past experience on Cray Vector Machines, I can pretty surely say that if your code is vectorizable, you gain huge speed up.