mlocate has replaced slocate as the default locate implementation after a short period in [testing], and an even longer one in [extra]. This was requested on the bug tracker a while back as FS4490
mlocate is used by Fedora at least and unlike slocate, no longer has the well known midnight updatedb crawl. In all other aspects it's compatible with and identical to slocate.
AndyRTR posted some performance measurements on the mailing list.
As described by the README: mlocate is a locate/updatedb implementation. The 'm' stands for "merging": updatedb reuses the existing database to avoid rereading most of the file system, which makes updatedb faster and does not trash the system caches as much.
The locate(1) utility is intended to be completely compatible to slocate. It also attempts to be compatible to GNU locate, when it does not conflict with slocate compatibility.