Since now updatesearchindexsolr.php is deprecated, we should make sure that the main script is good enough.
A problem it has: it only calls a commit when the index is done.
It should do a commit every N objects instead, maybe once every 50?
The updatesearchindexsolr.php script (eZ Find) considers the CommitWithin setting.
updatesearchindex.php does not considers this setting, but it should, and instead, it only commits everything at the end of the execution, which may also lead to scalability issues.