We haven't spent much time optimizing OMP yet, as it's relatively new software. That said, it's written with the same general standards as OJS, so I don't expect we'll have scalability problems. Now that there are a few installs with good amounts of content, it's a good time to start analyzing performance.
I suspect http://langsci-press.org/
doesn't run different code -- I suspect we would've heard back from them if they were e.g. changing queries. My suspicion is that their server is configured differently. The key things to look at are...
- Server API (SAPI): mod_php, CGI, FastCGI, ...? While mod_php is fastest, FastCGI offers a better balance of performance and security on shared servers.
- Opcode cache? There are XCache, Zend Optimizer, APC, and others; I've used XCache with good results. Make sure the resource allocations are sufficient.
- MySQL caching/tuning? Some of the default MySQL configuration settings are a little low. Database tuning is an art all its own.
- Server RAM. Insufficient RAM will cause swapping, which will badly degrade performance.
Of course there are other considerations, but these are the top ones that come to mind.
Those aside, there are probably some specific areas of OMP that will need code optimization. The most common suspects are slow MySQL queries; these can be identified by turning on the MySQL slow query log.
If there are specific parts of OMP that are performing badly, that's also useful information.
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