If you're like me you probably have a lot on your hobby plate and in the pipeline at any given time. For example right now I have my Imperial Guard army I'm repainting, my Traitor Guard sitting in boxes waiting to be created, my Sons of Horus begging for a little refurbishment, various Reaper, etc. figs sitting around, a nascent Death Guard army looking to infect me...well, it's a lot.
How to get it all done? Sometimes when I'm doing my hundredth left boot or my eyes start to bleed from painting detail or the glue fumes from assembling start to get to me, I need a change. If I stick to the task at hand, I could make progress, true, but it would be slower, lower quality progress and I wouldn't have fun. And this is supposed to be a hobby! So in order to still make some progress while garnering some level of enjoyment, I'll switch gears.
Tired of working on infantry? Work on a vehicle.
Tired of working on rank and file? Work on a character.
Tired of army X for a while? Break out something else.
...or work on terrain, objective counters, whatever. The key is to find something you'd be more enthusiastic about doing, and do that instead. You'll be happier, and while your progress on the original task may be slowed somewhat, your overall speed across multiple stages or projects will end up faster.