It's certainly been a widely accepted notion for some time.
It's worth noting that it's not that hard to cook up circumstances where that might actually be detrimental, say, a slow secondary drive that's been installed on the slave channel of an IDE with a CD-ROM as a master, or somesuch.
For some time I was juggling that accepted notion with the advice to run BFD on a separate drive from your audio files, since I had my notebook's internal 7200 drive (with the OS, app, AND audio data files at the time) and then my external USB2 drive.
After all the negative stuff I heard out in front of USB2 (and as a relatively happy Firewire user, I'll admit I might have been a little biased
), I was thinking of the drive as just an archival/backup drive. But it's actually respectable enough. I ended up moving my audio data to it -- more because of space considerations than anything else -- but it has to be said that I just don't do high track counts, 30 or so is probably the most I ever do, though I stopped actually counting a few years back, as a rule.
What kills me
is not track count but virtual instruments and nasty CPU sucking plugs like convo reverb...
I told myself I'd stop at just one and freeze all my tracks and... but then it's so damn convenient to have them all running live, even if the CPU meter keeps flying up into the pink...