scan time linear in time since last update?

I think this is probably actually a UNIX / filesystem question, but I'm trying to understand why sometimes
unison (with fastcheck on) takes a long(er?) time to scan my filesystem.

Here are my questions:

1) My assumption is that--all else being equal--the scan time should be linear in the number of files. Is
this correct?
2) In addition to this, it also appears that the scan time is related to--linear in?--the time since the last
update.  This suggests to me that the filesystem is scanning a log-based system to determine which files
have changed since the last update. Is this correct? FWIW, the times here are on the order of 4-5 minutes for
a tree with 40K files.

As I said, this is probably mostly a UNIX / filesystem question, but I'm curious to know whether my intuition
and observations are correct. If so, then I can presumably speed things up a lot simply by tarring a bunch of
rarely-used trees into single big files.

Thanks!

John Clements

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