Say, do the boilerplate images always present on every Mediawiki page have enough Expires and/or etc. headers to keep them cached around the network for a long time? (Can't tell here offline with images turned off anyway). Seems like optimizing that would need messing with the apache etc. configuration as it is beyond what one can do with php as they are <img src=...>. I.e., well at least we will send "304 not modified", as we (standard Mediawiki installation) are not going to go further to eliminate them asking in the first place... but as you can see, what do I know.