As for the checksum recalculation, you're right: it does take a lot more than 30s to compute all the numbers -- your 1h estimate seems to be right. The <1min time was related to a partial rescan, sorry for the noise.

So, what's the trigger for this 'full rescan' to happen? New files on the directory? Also, must it really recalculate all checksums, even if only a couple of files changed? Couldn't it do it only for those files for which name/size/timestamp changed since last run?

(I don't expect you to necessarily know all these details, this is actually addressed to Limewire developers)