Re: What's the problem?

[Richard Stallman]

    By doing the computation in chunks, like stealth font-lock does, for
    example, you let other tasks get slots of CPU time while the long
    computation runs.

Gnus could compute the summary in the background, piece by piece,
somewhat like stealth font-lock.  (Not exactly the same, because
whenever Emacs is otherwise idle it should work full time on
producing summary items.)