Writing a feature film on spec means having the time to make sure a scene work. There's no immediate pressure on me, no deadline to make a decision about a line of dialogue or an approach to the scene. This leads to perfectionism - which is my curse & the reason I've worked on The Limit for most of this decade.
Next film I write, I give myself a year to complete it. One year, then I'm getting out no matter what state it's in.
Next project, I'd like to do something shorter, driven by attitude and immediacy, developing it completely out in the open.
As I go further into (re)writing a script, expect to increasingly enjoy the act of writing and for it to go faster.
Tips for specific issues:
If I build a scene up in my head as something big, tough to conquer, then I can approach it with rock-climbing-like determination. I can set the goal of finishing the scene no matter what ... and set up base-camps at certain sections of the scene that I would reach, rest and then move on from.
If I keep re-finessing details in a scene, I need to force myself to keep moving. One thing that works is pitching it again and again, letting the edits come naturally, through performance, not writing.