That's a weird bug. For me it always works almost instantly with the exception of long exposures where it sometimes takes 2x the exposure time.

Did you switch to Camera2 API in settings>Camera Api ?

Yes, I tried every improvement and API switch suggested for almost a month with no change in performance unless I went back to default everything, which then brought processing performance in-line with the regular native phone app.

And the Long Exposure was the primary thing I wanted it to work for.