Fast radio bursts (FRBs) are millisecond radio signals from cosmological distances. As they propagate, FRBs can interact with ambient photons and initiate a quantum cascade that can limit the electric field strength. This paper examines whether some observed bright and brief FRBs may challenge this limit if the source is not relativistic. The size of a static FRB source is estimated as R ∼ ct, where t is the time-scale of the FRB and c denotes the speed of light. But for a relativistic source moving at the Lorentz factor Γ, the size is R ∼ 2Γ²ct. Using an FRB catalogue, we plot the luminosity–duration distribution. Most FRBs fall below the limit for a static source, but two events have higher luminosity and shorter duration. This suggests these bursts may originate from relativistic sources, although more data are needed to confirm this.