Without taking into account security, which hash function is the fastest one among the following:
MD5, SHA-1, SHA-2 (for various internal size), Keccak, and other… ?
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The performance of the hash depends on the environment it is used in.
Keccak excels in ASIC type hardware designs, whereas Blake and Skein excel in x86 and x86-64 environments.
MD5 is still quite fast in software, but newer algorithms take advantage of SIMD instructions on newer processors.
There is also the question of performance on a per invocation basis or on a per byte basis for larger messages. Keccak also has different speeds that depend on the security level (and message size).
SHA512 operates on a maximum message size of 885 bits before more than 1 invocation of the hash is required to process it, whereas SHA256 has a maximum size of 447 bits. That means a hash of a 128-bit message will be faster on SHA256, but a hash on a 128MB message may be quite faster with SHA512. SHA512 is about 50% faster on larger messages, but can be half as fast on small ones. This type of performance will vary with CPU type.
Performance comparisons with multiple message sizes and CPU types can be found here: http://bench.cr.yp.to/results-sha3.html