You're probably not fully verified with Coinbase if you're having to wait 4-5 days for a buy to go through. Go into your account and provide them with the info they want and you should be able to do instant buys. I think they just require a credit card and bank account to enable instant buys.
I have tried going through Coinbase, but for instant buy they require a Visa CC (debit does not work) and I only have Amex and MC.