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The "Seek graph" approach was used here for many years. Once they were able to use the server to pair most requests using the most common time controls, it became extraneous.
Now, only the unusual custom requests get posted there.
5|5 was the least popular of the default choices, according to Chess.com CEO Erik. The top 4 choices- 10 0; 5 0; 3 0 and 1 0 accounted for 84% of the live games. The remaining 5 split the other 16%. 5|5 was the least popular, accounting for only 2.7% of games. (They have apparently added another 5 choices since the article came out last May so it may have an even lower rank now.)
If automated matching is very quick for 84% of all live games, do you really want to slow everybody else down so you don't have to wait a couple of minutes?