Todays fastest airliners are travelling at transonic speeds (Mach No. =0.90-0.95 range).

To fly faster they would have to be supersonic. Breaking through the Mach No. = 1 barrier requires tremendous energy, as the drag, aerodynamics, etc. change at this barrier. Plus there are operational constraints - due to the noise of the sonic boom, the Concorde only flew supersonic when over open water or uninhabited land.

The fuel penalty to fly supersonic is enormous. Airlines are already being criticized (vilified?) in the press for the greenhouse gases being emitted by aircraft - supersonic aircraft would only make that situation worse.