A question about overlays and performance.

[Jeff Norden <jnorden@math.tntech.edu>]

I've been working on an improved emacs mode for CWEB files.

The elisp manual says this about overlays:

  However,... overlays generally don’t scale well...
  If you need to affect the visual appearance of many portions
  in the buffer, we recommend using text properties.

I've been using text properties, but am realizing that overlays would make
things a bit simpler.  Is there a way to predict how many overlays will be
"too many" and start to slow thing down?  Does the configuration of the
overlays play a role?

A file with 200 sections would wind up with 400 overlays, but the layout would
be simple.  There would be "section" overlays that are disjoint from each
other, and "subsection" overlays each of which is contained in a unique
section.  Each section would contain at most one subsection, so a position
would never be in more than two of the overlays.  It would also be easy to
call (overlay-recenter) each time the cursor moves to a new section.

Thanks in advance!
-Jeff