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A general description is here.
There is an experimental new ‘configure’ option ‘--with-libdeflate-compression’ that uses ‘libdeflate’ rather than ‘libz’ to (de)compress R objects in lazy-load databases, if the ‘libdeflate’ library and headers are available. Typically tasks spend up to 5% of their time on such operations, although creating lazy-data databases is one of the exceptions.