When computing the flux in filters, np.trapz() become the bottleneck of the code. A large part of the time is spent on safeguards and on operations for nD arrays. However in our specific case we only have 1D arrays. Also we can cache some computed variables, for instance dx which only depends on the wavelength sampling. We do that employing the same key that is used for caching the best sampling of each filter in compute_fnu(). That way the caches are consistent. The key is based on the size of the wavelength grid, the name of the filter, and the redshift.