Blurs (and related effects like glows) are extremely performance intensive effects, because for every pixel, it has to sample a whole patch of other pixels and combine them. Sampling multiple pixels per pixel rapidly multiplies the memory bandwidth requirement. It's so intensive that a large-radius blur could end up lagging even on reasonably high-end desktop graphics cards.
The best way to deal with that is to downscale the image to a low-quality version, process the blur on a small image, then stretch that back up for display. (Typically downscaling alone will cause some blurring, but in the case of a blur effect, that's what you want and so it works out well with big performance savings.) However Construct's effect compositor is exceptionally complicated, and we don't currently have support for doing downscaling like that, so for the time being we're stuck with hard-coded blur radius which can't be increased efficiently. I think the effect compositor will be rewritten at some point in order to support WebGPU, at which point we'll look in to other upgrades like downscaling blurs.
So in the mean time, the option with by far the best performance will be to just pre-render what you want in to a Sprite and add it as a child with an offset (or a 9-patch for rectangular things as in AllanR's project). If you don't need a blur, you could also use a cloned sprite with a black color filter and opacity, which is still efficient since the color filter and opacity effects don't go through the effect compositor.