Real-time strategy games have existed for a long time, one of the first big eSports games was even a RTS game; the genre mixed well with MMO and has now become one of the most popular genres of online games around.
These games take place in real-time and MMORTS games are dictated by wait timers that limit how many actions a player can make; wait timers can be 24 hours long in some games and the player cannot do anything with that building or unit during that time.
Games of this genre are usually very similar and there are very few games that break away from the expected gameplay; the player takes the role of a leader and is placed in charge of a base where they need to manage ../resources and grow the base. Usually the player is given the role of King and must build a castle or a town.
There are many standard features for this genre of MMO:
The majority of these games are free-to-play or have a freemium business model; it's very common for games to offer players a way to skip wait timers if they are willing to pay for it. Most PC MMORTS games are browser based however the genre is also popular on mobile devices and many mobile exclusive games exist now.
While this is the slowest MMO genre, it does still offer a lot of content to players. There are hundreds of these games in existence and they can be found on both PC and mobile devices.