Excuse my lack of knowledge on the subject. I know MD5 has it cryptographic security issues [1] but I almost always assumed that the main reason it was a 'weak' hash was due to it's speed and how, consequently, rainbow tables we're readily available. If this premise is correct, wouldn't that inherently make Blake2 less strong/secure since its faster?
[1] http://en.wikipedia.org/wiki/MD5#Security