My TWS is set up to default to limit orders. So even when I'm taking the bid price on a sell (or ask price on a buy), my order goes through as a limit order. Unless you're talking big size, my experience is that the order will be instantaneous if the order is taken by TMBR or an ECN. NYSE orders can take a bit longer, but as I indicated previously, most SMART orders are executed rapidly. So I prefer SMART routing unless I have a specific reason for driving the order directly to NYSE. Again, I'm talking small size, round lots like 100 to 1000 shares.
If it's a limit order going long, you're getting filled at market. Perhaps that's why you've experienced speedy fills with limit orders?
Yes, in my experience, 95% of the time 'hitting the bid" with a limit order at the NYSE will be faster than a market sell. Note, this is true of the NYSE, not necessarily with the ECN's. I've done a lot of listed trading (1000's of orders). I've always used limit orders to get in and out, unless there was drastic price action against me and I needed to get out at any price. Then I would use a market order, and it almost always took longer, much longer, an eternity in fact, if I really wanted out. But I got out.