The progression of builds is:
Insider Fast = Pre-beta testing release. Bug fixes, new features, new bugs, all sorts of fun stuff going on here. Not for general release, these builds are for testing. They're weekly and sometimes even more often.
Insider Slow = This build will become the general release a week later if no major show-stoppers are found. In this case, it appears that the bug is fixed, so the next Insider Slow build will adopt the bug fix. These builds are monthly. Depending on the
cutoff date the fix will be either in the very next Insider Slow or the one after. In general, these are safe builds, but there could be something odd that slips by (like the bug under discussion here).
General Release = Comes out a week after Insider Slow, and is also monthly. This is the most stable, reliable build and should be OK for everyone.
All the builds are staggered for release to manage the load on Microsoft servers, so there isn't a precise date or time for any of the various releases.