opening speed for nnml groups

[Steinar Bang <sb@metis.no>]

I haven't done any timing of it yet, but it seems like opening nnml
groups get noticably slower, as the groups get bigger (with more
ticked or read articles in them).

I was sort of idly wondering where the speed hit was?  Is it in the
parsing of the .overview file?

I remember that slow exit on big or sparse nnml group, was a problem
when you did expiry on exiting a group, so I went for demonical
expiry, as well as demonical fetch and sort.

If the reading and parsing of the .overview file is the problem, would
it be an idea to cache the parsed info between visits? And update it
during fetch and sort operations?

Or is that too expensive memory-wise?