I don't see anywhere where they claim that it's faster simply because it's written in C++. They do mention that they make use of C++ to add low level optimizations that make queries faster and the memory imprint smaller, but any claims about performance in the readme are linked to benchmarks to back up their claims
It is unlikely that it is because of C++, however, we have conducted extensive benchmarking (which, by the way, is fully open-source and can be easily reproduced if desired). You can find more information about this at https://manticoresearch.com/blog/manticore-alternative-to-el....
Also, architectural changes. They describe how ES can't parallelize a query unless it's spread across multiple index shards, which has its own tradeoffs. Their query engine can parallelize a query on a single index shard, which means it scales much more linearly on more cores without having to make those tradeoffs.
C/++ actually make you write relatively slow code too, by default. Not to the extent of Java, but still there is HUGE room for improvement in libC and by extension the STL. I'm working on a slash-and-burn approach to the problem here:
https://github.com/cons-cat/libcat
https://github.com/manticoresoftware/manticoresearch/
https://db-benchmarks.com/test-taxi/#manticore-search-vs-ela...