Like I said, the quickling should always run back behind cover, and with its speed being 4x that of the players, it should be able to stay ahead of them with ease.Even if the quickling wins initiative (not unlikely), if the wizard has mage armor and shield the quickling will have to burn all it's attacks to take her out, then the cleric hits the quickling with toll the dead whilst reviving the wizard with a healing word. The quickling can only take one more hit. It still loses.
If players are less skilled it's the DM's job to adjust the difficulty of encounters accordingly. No CR system can do it for them.
All I am saying is that I want monsters to be of roughly similar strength in combat at the same CR, and the quickling clearly isn't. Just because you can imagine circumstances where quicklings die quickly doesn't mean anything.