For Natural Selection 2, it was mainly the gameplay logic that was Lua, all running on their bespoke C++ game engine called Spark. But yeah, modern Python and Lua can be pushed to high performance.

Link: https://unknownworlds.com/en/news/spark-engine-questions-and...

Ah my mistake - I know they ditched Source to pursue their own engine and thought they were going all in on Lua, but doing the whole engine in Lua would have been a struggle I think.