In their most recent developer address, Riot have given us information on the state of competitive play. It would appear that DuoQ is causing most of the problems found, and Riot are thinking of disabling it.
Currently in League of Legends, from a high-level viewpoint, the Solo/Duo queue is in one of the healthiest states ever seen.
Autofill is at its lowest ever global rate hovering between 0.5% and 3% of player games, while Queue times are at a 2 year low in many large regions and hovering firm in the small regions. Lastly, MMR accuracy is strong, where 99% of players are playing with those within a maximum of 2 divisions of their rank with a steady win-prediction accuracy to back it up.
Where all of this changes is when premade Duos enter the mix, causing the autofill percentages to skyrocket and MMR to become imbalanced.
To combat this, Riot are thinking of instating a Solo Only Mode. In order to match player intent and provide the healthiest competitive individual queue possible, developers say that they're re-evaluating their take on Solo/Duo and exploring a world where solo players play in solo queue and premades play in Ranked Flex or Organized 5’s modes.
However, the number of premade players that play League of Legends is quite high. To that end, Riot are still unsure what the exact implementation will entail, as the current Flex Queue is placed below the Solo/Duo one in importance by the community, and implementing a Solo Only Mode may alienate the large premade population.