A couple of years ago I wrote an mbox parser that was several times faster than Mutt's. It really showed because at the time my server was running on a Soekris net6501, my mailbox was a few hundred megabytes, and my habit is to invoke and exit mutt repeatedly rather than using a dedicated terminal window. Since then I've been running my server on a Xeon E3, and it's less bothersome.
I investigated patching Mutt to use the parser, but it seemed a little too much trouble for the effort given how many one-liners have accreted in the existing implementation. Using an index would make things more responsive, too, but the faster parser could provide competitive responsiveness into the gigabyte territory while keeping things simple, in both code and on the filesystem.
Is this a common enough gripe that it might be accepted and appreciated? My parser used Ragel but I could implement it again in plain C so it didn't have a build-time dependency.