It depends on the individual system, and as you expect there are systems where accessing ROM is equivalent to RAM speed wise. You mainly first began to see a s

It depends on the individual system, and as you expect there are systems where accessing ROM is equivalent to RAM speed wise. You mainly first began to see a speed disparity with full 32 bit systems which had a 32 bit data path to memory. It'd be common to still only have a 16 bit datapath to ROM, necessitating the hardware to split 32 bit transfers in two when accessing ROM. This disparity ended up diverging further as the CPU only had to hit the north bridge (or more recently the memory controller in uncore on the CPU die) but practically having to go all the way out to some of the slowest, legacy interfaces like LPC to hit ROM.