Biggest bottleneck? I don' think any ONE bottleneck is a big problem, but a combonation is.
first of all, we're not talking speed of human compared to computer, we're talking how fast the computer can execute it's commands, and therefor show the result onscreen.
in that case, CDROM's Floppy Drives, and Harddrives are the slowest DEVICES.
The slowest interfaces/busses, are the ISA interface (which is being fazed out anyway), and the PCI interface (max of 133Megs/second). The bus that needs speeding up most, would be the one between the north and Southbridge, especially if drives ever become as fast as RAM.
Why don't I count RAM in there? becuase it's simply not as big of a problem as the harddrive is. if the harddrive ran as fast as RAM did, we wouldn't need System memory, just a fair amount of cache for the CPU.