It doesn't take very long for the pills to dissolve into the water as long as you continue stirring the solution. Yes, you should crush the pills or it would take much longer for the pills to dissolve. You don't waste any oxycodone when crushing it, you just increase the surface area of the pill so it dissolves quicker. Coffee filters are what I used for my CWE, however I used to use paper towels. Paper towels have two disadvantages when performing a CWE: firstly, even if you wet the paper towel before filtering the paper towel will still absorb quite a bit of the oxycodone into it, and second since it filters the solution very quickly it also lets a lot of APAP through into the final solution. It is quicker however.
A coffee filter filters out most of the APAP, leading to a much clearer solution, however it is significantly slower because it. It's up to you which you want to use basically based on how much time you want to spend on it, but if you're doing 10-15+ pills I'd recommend a coffee filter because you do not want a lot of APAP in your final solution. Less than that and a paper towel would suffice.