It's actually not that much more computationally intensive, even doing texture RGB -> spectral wavelength conversion for each BSDF / light evaluation.
But you do get a fair bit more spectral noise in the image, but this can be handled by handling multiple wavelengths at once with SIMD and importance-sampling the wavelengths to use (Hero wavelength sampling).
Where it does actually make a lot of sense over RGB triplets is for volume scattering / absorption
(so more accurate hair and skin rendering for example) - calculating the mean free path with wavelengths is much more accurate than using an RGB triplet which is an approximation - however, this means to enjoy this benefit, you need to store the volume coefficients in wavelength bins (instead of RGB values), which uses quite a bit more memory.
But you do get a fair bit more spectral noise in the image, but this can be handled by handling multiple wavelengths at once with SIMD and importance-sampling the wavelengths to use (Hero wavelength sampling).
Where it does actually make a lot of sense over RGB triplets is for volume scattering / absorption (so more accurate hair and skin rendering for example) - calculating the mean free path with wavelengths is much more accurate than using an RGB triplet which is an approximation - however, this means to enjoy this benefit, you need to store the volume coefficients in wavelength bins (instead of RGB values), which uses quite a bit more memory.