Post-processing costs one full-screen pass each

Every post effect renders your scene to a texture and then runs another full-screen shader over it, sometimes several: bloom alone is a threshold pass plus a downsample and blur chain plus a composite. Each pass touches every pixel, so the cost scales with resolution rather than scene complexity, and the phone that handled your geometry fine will die on a three-effect chain at device pixel ratio 3.

The cheap alternatives are worth knowing because they are usually enough. Emissive materials plus a subtle single bloom read as glow. A vignette is one multiplication in the final shader rather than a pass. Baked ambient occlusion in a texture is free at runtime, and screen-space ambient occlusion is not. Render post effects at half resolution when the effect is low frequency, which for bloom is essentially always.