Simultaneous submissions make sense for agents and NY pubs, I think, but I don't think the process of simul subs works in epublishing. I know people who have done it thinking if they get an offer from one publisher that they can email the other pubs and get their MS bumped to be looked at sooner, but that rarely happens. With agents that's how it works (most of the time, anyway), but epubs don't tend to do it that way. They're more apt to just ask if you're going to accept the offer, so they can then remove your MS from their to be read pile completely.
It's also another reason why I think it's a good idea to pick your top pubs and form a relationship with them as an in-house, repeat author, because then you don't need to go shopping around and get read a lot faster than slush.
As to the wait time at DSP, it's been years since I worked with them, but a friend of mine is a regular author there and gets responses to submissions within days. Like, 3 days or so lol. I have no idea about people submitting through slush, though. I don't know anyone doing that. But once you're in, apparently the response time is lightning quick, so there is that.