It is lz4. While all that you say about disk being slow makes sense, measurements show that in this specific situation disabling compression improves performanc

It is lz4. While all that you say about disk being slow makes sense, measurements show that in this specific situation disabling compression improves performance by a little. My hand-wavy explanation is that freeing CPU to doing compilation rather than [de]compression is a better use of it, with all the uncompressed disk writes probably ending up in the system's file cache.