On a related note: When I indexed the whole English wikipedia last year, I was surprised, that it was possible to have a JSON version of it indexed[1] and searchable within half an hour on my laptop.
That being said, when it comes to indexing wikipedia, the indexing can be done well across multiple threads internally by elasticsearch. Multithreading the reading/parsing isn't a huge win. Doing decompression in a separate thread is however.
Yes, ES uses multiple threads nicely. But as you move to 32 or 64 cores - in my experience - a single threaded client won't keep ES/Lucene busy enough.
With SOLR, it's similar:
> Sometimes you need to index a bunch of documents really, really fast. [...] The solution is two-fold: batching and multi-threading
[1] Using parallel bulk indexer for ES: https://github.com/miku/esbulk