Comment by TylerGlaiel

Biggest place for improvement performance-wise I can see from this is that you aren't taking into account cache coherency here. Horizontal blurs with this method are going to be ripping fast, but vertical blurs are going to constantly cache miss. There's a number of ways to potentially fix this, the fact that you only need to store 3 sums for one line means you could do a few (64) columns at a time and just store that many distinct sets of sums (sorta like doing 64 columns in parallel, just making sure to use the data thats in the cache while its still in the cache)