I feel that HD speed is one of the biggest bottlenecks in my Amiga setup. Sure, the Z-IV busboard and the CV64/3D can hardly handle Doom. Sure, the 060/50MHz isn't exactly top of the line anymore. But all file system operations are a real pain. Sure, my HD needs a good defragmentation but at about 1.8-2.0MB/s (SysInfo) that's what slows things down.
I remember when Amigas booted in just a few (around 10) seconds. My Amiga takes around 30-40 seconds to boot, then load ToolManager graphics, backgrounds... All file requesters are dog slow and thank god for MCP's font cache.
I'd switch to SCSI any day if I could afford it!