[edst]

You're right -- the re-rendering could be an issue for larger boards. In this case, you could build a new version of mimetex.cgi that uses the -DCACHEPATH=\"path/\" argument and still cache the images locally. This probably makes sense to do whenever the workaround is needed.

On that note, if you're building your own mimetex.cgi anyway, I'd recommend using the -DREFERER=\"domain\" argument too. This will add some level of security so that others cannot make use of the engine if it's located in a public area of your server.