Welcome back Stephen! One trick I use to optimize my large scale blur's is to use 2 Blur's. First blur is 1/4 resolution followed by a second, full resolution blur (in pixel space rather than world) with a distance of 4 to match the 1/4 scale difference of the previous blur. This usually gives me indistinguishable results but is incredibly fast. For example, I'm currently replacing a blur which takes 4:47 to render at full resolution (8192) on 40 cores and 210 GB of memory. The optimized version takes 9 seconds.