I want to inform you that I have read this message and have been doing some research. I don't have a Win10 machine I can devote to testing/developing. I can occasionally try one briefly, but it doesn't have this problem you describe in the storms.
Given what I've uncovered on the internet, Microsoft even specifies that a certain update for Win10 can cause DirectX 9 performance problems but they kept it in place for "security." LOL
Anyway, some game developers in various forums have uncovered some specific device options at DX startup that might cause this, so I've put an additional log message in the game engine launch to capture the values used. I've also got a few other things I will want you to try, but I need to get my volunteer testers to devote some time before I upload the engine modifications. "We" are one software developer, doing this game part-time because I have a full-time job, so give me a few days to get this sorted and I will upload to Itch, then I will reply back with some things I want you to try.
In the meantime, find a storm in ERAS, and save the game there...keep the save file file handy because I will want you to try some options using that save game in the storm.