Don’t waste time on sentences if you haven’t fixed your paragraphs. Don’t waste time on the paragraph if you haven’t fixed the page. Worry about big picture first, then the details.
If you spend a monumental amount of time tweaking sentences and then cut the whole scene, you will feel like an idiot.
This is true in editing as well as writing. Get the story structure right, then start worrying about what’s happening in the scenes.
What you don’t want to do, ever, ever, ever, ever, is spend one second on something you’re going to throw away later.