For a back of the envelope calculation we consider the uncertainty principle Δx*Δp >= ℏ/2 Δp = Δ(mv) >= ℏ/(2Δx) m = 9.1 * 10 ^ -31 kg (mass of electron) x ~

For a back of the envelope calculation we consider the uncertainty principle Δx*Δp >= ℏ/2 Δp = Δ(mv) >= ℏ/(2Δx) m = 9.1 * 10 ^ -31 kg (mass of electron) x ~= 1 * 10^-15 m (radius of proton) ℏ = 6.6 * 10 ^ -34 kg m^2 / s (plancks) Δv >= 31 524 512 m /s Which is about 1/10 the speed of light. There isn't a true cutoff for "relativistic speeds" but in general, 1/10th counts.