I doubt "expedite the decision" is incorrect. I have seen people and even government website using, "Expedite decision"., "to expedite a decision" and etc. So I am quite sure decision can be expedited. As for your: "The decision-making on the painting to be expedited to avoid the works’ execution in wintertime.", my suggestion is: "Please expedite decision on the painting without causing execution delay in winter". Just my 2 cents worth.

The decision-making on the painting *was* to be expedited *as* to avoid the works' execution in wintertime. This is very formal and almost reminds me of prose. I can only imagine someone using this kind of language in formal writing, unless someone wanted to sound very intellectual.

Here are a few options for you. Hope this helps! A. To prevent any delay on the painting, a decision will be needed by (enter date). B. It would be beneficial to make an early decision on the painting to avoid having to start the work during the winter. C. If you can decide on the painting by (enter date), this will prevent the work from beginning during the winter. D. To help expedite the process, the painting should be decided on by (enter date). This will prevent the work from starting in the winter. As a native English speaker - "expedite the decision" makes sense but sounds strange and a bit like the scene of a sci-fi movie. I would not use this in formal writing. You could say "you need to be expeditious in your decision" - but that is very formal and not commonly said. Best, Mari

I've changed my "comment" to an "answer"