The principal factors which limit the bandwidth and noise temperature of the free‐electron bolometer mixer have been calculated. The minimum mixer contribution to the effective noise temperature is found to be 33Tlatticeunder the conditions of rf match, IF match, and constant current bias. This result is independent of the electron‐lattice energy relaxation time, and can be achieved without significantly degrading the information bandwidth. The best experimental values from the literature are found to be near this predicted performance.