How does this app pull off fast, high radius HDR
At CEDEC and GDC in the early 2000s, Masaki Kawase has presented a series of fast post-processing based lens effects, including large bloom.

This presentation from GDC 2003, Frame Buffer Postprocessing Effects in DOUBLE-S.T.E.A.L (Wreckless) (see slides 15 and upwards: "Bloom"), gives a first version of the bloom. It consists in doing a Gaussian blur by sampling at exponentially increasing distances.

In this presentation from GDC 2004, Practical Implementation of High Dynamic Range Rendering (see slides 44 and upward: "Glare generation"), he updates the technique. Instead of varying the sampling distance, he uses downsized versions of the original image, using a carefully crafted equation to achieve a large yet spiky Gaussian blur.