For the early versions on Archimedes hardware, yes. Later versions had increasing amounts on disk, which increased the boot time from 'nearly instant' to merely

For the early versions on Archimedes hardware, yes. Later versions had increasing amounts on disk, which increased the boot time from 'nearly instant' to merely 'fairly quick'. Although recent versions of RISC OS are now entirely softloaded, the increased speed of the Raspberry Pi and other modern hardware platforms has cheerfully reduced boot time again: by modern standards RISC OS is tiny!