I don't think the performance is the biggest issue. However, the results
of current (i.e. Gimp 2.6.x) downscaling are.
In Gimp 2.4 I could use the "Cubic" Option which resulted in a little
blurring, but that could be fixed with a judicious use of the "Sharpen"
filter. So in the end, it yielded a lot of control about the final result.
The new Gimp 2.6 "Cubic" option usually yields too bad quality to be
considered (I won't even begin with "Linear"). This also applies to
"Lanczos" in some cases, but at least that option is somewhat useful.
In any case, the results look like Gimp 2.4 with an overdone Sharpen
filter applied (20 or more) or so which results in ultra-sharp
downscaled images. Though it is acceptable for several typical cases and
may be what people expect of a default downscaling behavior, that is not
always what is required by the artist.
So, yes, different scaling options are a good idea. And they also should
yield usable results (do not sharpen automatically, and if that is done,
it should be an option as well).
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