You need to change the filter logic, e.g. use filter separation to split up your filter into two passes (instead of n*n you have only n+n texture lookups).

Then try to utilize the hardware linear filters, that is you can access with a single texture lookup the linear sum of 4 texels or atleast 2 texels, this is very useful for blur filters.

With this simple tricks you can reduce a naive blur filter implemenation (9x9=81 texture lookup) to just 10 lookups.

You need to change the filter logic, e.g. use filter separation to split up your filter into two passes (instead of n*n you have only n+n texture lookups).

Then try to utilize the hardware linear filters, that is you can access with a single texture lookup the linear sum of 4 texels or atleast 2 texels, this is very useful for blur filters.