Hnns Hlgr Rtz is a user on octodon.social. You can follow them or interact with them if you have an account anywhere in the fediverse.
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Ok, so I implemented that Gimp-style slur filter; it takes 5 minutes per frame; video has 40K frames. I just need to render for five month 
I implemented the UGen directly, takes 5 seconds now, so that's viable for the video project, and I still have a variable kernel definition, not a bad compromise.
I checked that into FScape v2.14.2
https://github.com/Sciss/FScape-next/commit/49d27925274e001eef2309d18798f107c35fdd09
I guess I could speed it up implementing a DelayN UGen instead.