Ideas are like fresh food in that they haven't been processed or manufactured yet and because of that have no inherit preservatives built into them. This means they expire relatively quickly. The longer you have an idea sitting there, the less appealing it becomes to work on. So it is imperative to work on ideas as quickly as possible and not to let them rot in idleness.
From my experience, I've noticed that effort is a function of energy level. So when I have low energy, things tend to take more effort. Energy is a function of motivation. The freshness of something is a part of the motivation to work on it. The longer an idea has been sitting there, the less motivated I am to work on it. So something that is relatively quick and easy to do when I initially thought of it becomes much harder once it has been sitting on some list for a few weeks. I believe there are two reasons for this.
The first is that though the task might not be intrinsically harder, the world has been changing in the meantime and you're no longer certain that the idea is still valid in the new world. Not only do you have to download the idea back into your head, you also have to check if the context surrounding it still applies. Is that fantastic fitness app that you thought of 3 months ago still the unique snowflake you thought it was or has someone already built something similar? Do you still plan on taking those cooking lessons now that you have this brand new pressure cooker or will some youtube videos suffice? The work before the actual work has now increased. The longer you wait, the more potential for the world to change and the more work you have to do to ensure the idea is still valid.
The second reason working on things after the newness is gone is harder is precisely that humans are attracted to shiny new objects by nature. The longer you let an idea sit, the more potential there will be for other newer ideas to grab your attention. Before you know it, weeks or months have passed since the conception of the idea. When you finally come around to work on it, it's no longer the sexy thing to work on and feels more like a nagging chore than the brilliant, infallible masterpiece that it once seemed to be. It's said that time ripens all things, but time also spoils all things that aren't consumed when ripe.
Perhaps the worst side effect of letting ideas sit on the shelf is that these old ideas often make it harder to have new ideas. Since there's a part of your brain that's keeping track of the old idea, you tend to compare new ideas to the old ideas in order to prioritize them. But at the idea stage, prioritization can be tricky and the longer you've had an idea the less willing you are to let it go. You become invested in it to the detriment of all your new ideas, which could possibly be much better than the old ideas. It is very much letting food sit too long in the fridge -- when you open the fridge you might think you don't need to buy any new food because it is so full, but if it is full of rotten food, the fridge might as well be empty as you'll be starving either way.
My preferred way of dealing with this is to make it a habit to put each idea to the test as soon as possible. If not with a direct product, then at least with a proof of concept. Making something real removes the romantic subjectiveness that often surrounds theoretical ideas. Your judgment of it becomes more objective as you are able to see any potential downsides in the flesh. But besides its judgment, the main benefit of working on an idea right away is keeping the momentum that comes with novelty. The quicker you get started on an idea, the faster it gets implemented. At the outset your motivation to work on an idea is pretty high and thus your energy levels are high which often translates to faster, better work. This in itself can often prevent idea rot as many perfectly good ideas have perished simply because they were left to sit on a shelf for too long.