The slowdown is relative to other metrics related to computer performance, I imagine.
Also while reliability and capacity has both increased, capacity has far outstripped reliability. So that while today's drives are generally more reliable then older ones (as in bad/corrupt blocks lost per GB) the chances of you losing part of your data is much higher simply because there is so much more of it.
This sort of stuff why online fsck and scrubs (reading in data and comparing it to checksums to detect and correct corruption) is so important on modern file systems. Previously the only people that needed to care were ones that could justify the expense of purchasing big SAN devices and whatnot.