> 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

No, for hashes, you definitely want speed; that's not the problem with MD5. For applications where speed is not appropriate, such as password hashing, you want an iterated hash of some kind. (Disclaimer: as with most crypto, you want one of the existing implementations, not something custom.)

The cyptographic security issues with MD5 concern breaks to the hash algorithm that make it possible to find hash collisions more easily than the number of bits in the hash would imply. A 128-bit hash should require 2128 hashing operations to find a collision against an existing fixed value. (Finding two independently chosen values with the same hash is far easier, due to the birthday problem: https://en.wikipedia.org/wiki/Birthday_problem ) MD5 has algorithmic weaknesses that make it far easier to find collisions, to the point where a current system can find some types of collisions in seconds.