"least performing" is correct and usable in written English.
You can use it to refer to something that has not been doing as well as other things or people. For example, "The least performing students were sent to the counselor for extra help."
Q. Was she at least performing it?
"It [the government] appears to be more focused on the children whom we serve relatively well, the brighter kids, and not focusing on the central problem we have which is doing a better job for the children at the bottom... Why isn't the focus on the bottom 20%, the poorest and the least performing?
The least performing internally cured samples (30% partial replacement and under IC4 condition) had about 11% higher modulus of elasticity than IC4-control samples.
Steinernema sp. RW14-M-C2a3 was, overall, the least performing (4 ± 2% of grubs killed across concentrations, Figs. 1 and 2).
The dat set size of the best performing model of the receptor 5-HT5A with PVE value 0.87 is 302 molecules and least performing model of the receptor HRH2 with PVE value 0.32 has 385 molecules.
At the same time, many of our buildings have long dreamed of becoming signs, or at least performing a credible imitation of them.
The work, notable for the lovely theme in the Larghetto, is probably the least performed of Beethoven's symphonies.
Their combined advocacy brought one of Puccini's least performed works back into the repertoire.
Then why didn't Schnabel at least perform his own music?
The Seventh is still the least performed of the cycle, but Abbado's recording reveals its visionary, colouristic genius.
Then the surplus went away, and the economy did not, to say the least, perform very well.
