Nintendo Switch

Simple version: it’s not actually “loading” anymore. It’s loading (pretty fast), decompressing (not very fast), and in many cases precomputing (potentially very slow). Current cartridges are storage for the same kind of data that was found on CDs and other media, just in solid state form.

OG cartridges, as was pointed out by someone else, were not doing any of those things. They weren’t even always exactly just storage, they were basically a black-box addition to the logic board. In fact, some cartridge games included entire additional processors that were looped into the logic to provide additional processing power for graphics or audio. That’s what Super FX on the Super Nintendo was: a math coprocessor in the cartridge that helped handle 3D graphics.