Looking back over a week or a month, you can figure out if you do your best work in the living room with the kids, a coffee shop with no wifi or your desk. You'll also know if you do best mornings or evenings, before or after walking the dog....make the best of that knowledge, and make it work for you. Sometimes, we need all the help we can give ourselves!
Some parts of a story we approach with eager excitement, others bits we know have to go down on paper but the scene doesn't exactly excite us. The only answer is to try and drum up some excitement in how you approach it get on with it. Try and change the scene somehow. Give it a kick up the backside, turn it on its head, do it from another POV, anything to make the scene live, so that you have some pleasure in writing it.
Get rid of the idea that it's a sin to write fast. Some of our famour authors write really fast, though there are others who take years to finish a book. As we've said before, some of the best scenes come really fast, for whatever reason, and then what are we tempted to do? Get out the proverbial red pen and go through it until we've taken all the life out of it. Knowing when to leave well alone and move on to the next scene is hard to learn, but it's worth it. Believe that fast writing can be good writing and leave it alone. You can always go back to it later, when you're on a second draft.