I heard/read all the horror stories associated with Sigma's processing software before I bought a DP3M. Yes it is slow, ponderous and increases your workflow quite a bit.
You have to save the raw files as a 16 bit tiff and then work on it again in LR. I have a pretty fast computer and load my sigma files from a SSD.
For me it takes maybe 2-3 seconds to open each file. the software shows a full screen preview immediately and then takes some time to open an editable version of the file. And then if you want to see the image full size, it takes an additional 2-3 seconds.
On a slower computer the wait time might be completely unacceptable. It's definitely clunky.
But for me, the images I'm getting out of it are like nothing I've seen (I also shoot with a Nikon D800e, so am quite used to working on files which carry a lot of detail. But the Sigma throws up similar files, albeit with lesser resolution, but in astounding color). It's almost a shame one has to save the images in sRGB for the web.
About the processing itself, I actually love the sliders in the software. You can really push the file around and the sliders keep everything in check. There are very few tools to work with in the software, but the stuff that is in there, is among the best I've used. I might be the only person on the planet who actually kind of enjoys using the software. The results are so good that it keeps me interested.