Poor performance following ShadowAssist.CacheMode change #3616

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### Bug explanation

It seems that specifically following commit #3616 to change the default ShadowAssist CacheMode behaviour, I find the performance of our WPF app to be a lot slower and laggy than it was before, particularly when it comes to animations such as those in the DialogHost flyouts. It's particularly bad after resizing the window or in full screen.

I'm trying to figure out the best way to replicate the old behaviour. I've tried to set a `CacheMode `with `EnableClearType` and `SnapsToDevicePixels` but it seems like I have to set this all the way down and just setting it on the window or main containing control doesn't work.

Would appreciate any guidance on this. Has anyone else had a similar problem or know the best way to handle the CacheMode property globally? Is that even the right approach or should it only be set for troublesome controls?

### Version

5.2.1

Start by reviewing commit #3616 and reproducing the slowdown in the WPF app, focusing on DialogHost flyout animations after window resizing or entering full screen. Compare the CacheMode behavior with the previous version and determine whether the issue is global or limited to troublesome controls; done requires a reproducible cause and a documented handling approach.

Written by the indexing model from the issue text.