Using
Monstrous Races point-building system...
Fey (0.5 pts)
Small (-1 pt), this means based walking speed starts at 25 ft.
Speed 60 feet (7 pts)
Darkvision, 60 ft. (2 pts)
Dexterity +2 (2 pts)
Opportunity Attacks vs. Quickling Have Disadvantage (?, maybe 2 pts?, comparable to a much weaker
blur spell but always active) -- you can really play up the strangeness of this, perhaps with transparent overlays of the quickling occupying multiple emotional states in a sort of shutter-frame-rate or stop-motion animation
Approximately 12.5 pts, possibly comparable to a Drow, though depends on how its special power is evaluated
Typically, ~10 pts is the target for race building, so it's at the upper limits
We could reduce its speed to 40 feet (-2 pts) and give it Bonus Action Dash (+1 pt), and it would come out to approximately 11.5 pts.
Alternately, you could have it start at 40 or 50 feet speed, then gain speed as it levels up, perhaps +10 feet for every additional proficiency bonus increase (i.e. at 5th, 9th, 13th, 17th).
You could also increase its speed when specifically taking the Dash action by some amount.