I don't know for sure that this is relevant to your situation, but it may be of interest to others anyway.
A little while back, a bit frustrated with the slowness of my dh Gallery, and realising that I hardly used any of its facilities, I decided to automate a static html+image gallery.
This was only for my own and family's use, so I haven't gone to the nth degree with research, but basically, the approach I've settled on involved:
- setting up llgal (http://home.gna.org/llgal/) on my dh site
- configuring a dh cron job to run llgal once a day
- setting up a cron job on my home machine to sync my laptop's photos with a dir on my dh account
With those in place, I now have a very simple gallery. The upside: very, very fast, with little drain on the server; entirely automatic (any new photos just appear there with no intervention on my part). The downside: no facilities to speak of (no search, resize, etc); no real authentication (I do use apache basic authentication, but that's limited).
My site is tiny and personal, so I've no idea how this would scale. Certainly for a lot of photo's the first run of llgal (which uses convert) would take a while. But after that it's incremental.