There have been some changes to file loading to speed up startup time, and autosave has been added to edit mode. With those two additions affecting critical features, it is possible that some detail was missed (maybe a particular rare tag in simfiles doesn't work anymore, or maybe files aren't saved or loaded correctly)
Thus, this release candidate build is so that people can test things on non-critical systems to make sure simfiles that worked fine in 5.0.6 also work fine with the changed loader. (non-critical systems just means don't go stick it on an arcade machine where someone might get pissed if a song doesn't load or there's a crash)
If you encounter any problems related to song loading or saving with this build post on the forum or submit an issue with as much info as you have. If it's a particular song that doesn't load right, that simfile will probably be needed to solve the problem.
If there are no serious bugs in this build, the next release will be in 3 weeks (April 1st), along with any changes that occur between now and then. Work on improving startup time is still ongoing.
Note on loading time
Because this release includes a cache version bump, the first time you load will take much longer than normal because it has to rebuild the song cache. After that, it should be measurably faster. People using SSDs or on modern hardware or with small song collections might not be able to see any difference in loading time.
Github release page