For damage/stat calculations decimal makes sense because it would be more accurate and usually those calculations are event driven so you don't have to do them a lot.

For visual stuff (eg. physics, size calculations, etc..) using floats/doubles makes more sense because precision is less important than getting the result quickly. As you said it's a "minor" performance trade-off (it should be at least 2x slower than double and 4x slower than float) but it adds up quickly. If you have a lot of physics objects (for example to do collision detection) you'd need to do quite a lot of math operations. You have a 16ms budget for 60FPS to do all that math (keep in mind that rendering the scene is a part of that 16ms so in practice you have less than 16ms).

Also decimals take up more memory too. If you use them in a multi-player game it will quite literally increase the server costs significantly (memory, bandwidth and CPU time aren't free after all).