without changing the schema of the DB, the first thing to try is caching. If the inner entities cache well, gains will be substantial.
Maybe the wiki is not uptodate so you should check the jira issues, namely solr-2382 and maybe have a look at solr-2948 too.
A second path could be trying multithreading DIH, but it's more tricky. At one point this was optional, but later was removed cause it was buggy, and I think now there was some jira issue trying to reimplement it, try look it up, but I recommend caching first.