If you want a even crop around the whole image, then use the Border filter in your Video(Built-in) filters bin. This is a one-pass filter, so it will take the length of your clip to process.
If you want to do it another way, so you have a bit more control, then create a matte on your clip (animate it if you want) then just XBLUR the alpha channel. Don't add softness in the Matte tool. It will take longer to process the matte out, and you'll get a better quality blur with the XBLUR and it's realtime!
Hope that helps,
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