We present the detailed design and performance simulations of BIFROST, a multiplexing indirect neutron time-of-flight spectrometer at the European Spallation Source. The instrument allows a neutron bandwidth of Δλ = 1.74 Å to reach the sample. The polychromatic flux can reach 6 × 109 n/s/cm2 while retaining a relative energy resolution, δEi/Ei, of around 3.5% at Ei = 5 meV. A fast pulse-shaping chopper allows for flexibility in the primary spectrometer resolution. For an opening time of 0.1 ms, a relative energy resolution down to 0.2% is achieved for Ei = 12 meV. The secondary spectrometer consists of 45 analyzer arrays, populating 9 scattering angle channels in the horizontal scattering plane, each of which covers 5.2° in scattering angle. Each channel hosts 5 analyzer arrays reflecting neutrons of fixed energies Ef equal to 2.7, 3.2, 3.8, 4.4, and 5.0 meV. Utilizing the prismatic analyzer concept, the back-end geometry allows a secondary spectrometer energy resolution in the range of 0.02-0.05 meV. The unique design of BIFROST offers not only an unprecedented neutron flux but also the ability to adjust the energy resolution by more than one order of magnitude. Focusing on the horizontal scattering plane, the spectrometer is ideally suited for extreme environment studies and for studying samples much smaller than 1 cm3. The drastic increase in measurement efficiency, compared to current high-flux spectrometers, has a particularly high impact on the fields of quantum magnetism, unconventional superconductivity, and functional materials.