I'll explain it in an easy way. If you have 1g of pure 100% oxycodone powder and 1g of pure acetaminophen powder. You mix the 2 together really well and obtain a powder mix, of 2g right? If you dissolve that powder into cold water your right, opiate dissolves immediately in less than 0.1 seconds, the reaction is extremely fast even in icy water.
What you perhaps didn't consider is that crushing a pill containing 750mg of APAP and 30mg of opiate is not a mix of 750/30, because it contains a chemical called a builder, made to hold the pill together in a solid shape. The builder will prevent the opiate to fully dissolve in cold water unless you shake the crap out of it. It would be more wise to destroy the builder through a heat treatment first...erowid does the same, it's not a coincidence. Some pills have really tough builders...it can prevent even 20% of the opiate to be released but it's usually lower than that.