This is perhaps the most obvious and “not very helpful” post for quite a few people, but for those who experience this issue, it’ll save the day. While doing a test index routine of around 6 million documents, things would get really slow at the moment I passed 1 million documents in the index. Weird. Optimizing didn’t help, as it died with an exception after a while.
The reason?
Not enough free disk space. Solr was indexing to a different partition than I thought.
Solved everything.
Thanks for this post. I didn’t find the reason that our solr indexing slows down after a while. Reloading the core helps … maybe a cache issue (to be investigated).