In a bit of a shocking move, Mojang released Minecraft 1.10 less than four months after 1.9. As always, the servers aren't ready for it yet, so be sure to stay on 1.9.4 for now to play with us.
Considering that this is a much smaller update with fewer breaking changes, the upgrade process should be faster. There are still some issues to handle properly, most notably the availability of the new Nether blocks. I don't have any insights on this as of yet; more news will follow when available!