Depending on how much you need to convert to USD.  Coinbase has a daily buy/sell limit of 50btc at level 2 (highest), but it's also nearly immediate (normal 2-3day clearing w/ banking system).  I recent sells were credited to my bank account in 2-3 days. 

However, the downside is that they charge a 1% processing fee, and you can't hold any USD balances, so all sells/buys are direct withdraw/deposit to your linked bank account.