Does installing a bunch of unnecessary fonts slow computers down?
The reason that it is recommended is that Windows (at least, not sure about Mac and Linux) has to load font information on boot. Not only can this slow the boot process (though I don't think you would actually notice this on a modern computer) but, more importantly, each font requires an amount of in-memory storage. This is then no longer available to other OS processes and so can slow down the OS due to paging.
So on a modern computer with plenty of memory and a fast drive, you won't notice any difference under normal circumstances.
The main performance hit is not the fonts on their own, but the extended load times for applications using them (Word, Excel, Corel etc.).
Graphics-Unleashed
SourceDaddy
From my own experience, I have to say yes, installing many fonts will slow down a system. My own anecdotal evidence is as follows:
In early 2011, I wanted to see if there was any truth as to whether a lot of fonts slowed down a system. To test it out, I used FontFrenzy on a Windows 7 machine to unload all but the fonts installed with Windows 7 by default. In all, I disabled about 250 fonts I had accumulated. That being the only change I made, my boot time (from BIOS screen to useable Desktop screen) lowered by 6 seconds (from 50 seconds to 44 seconds). Programs like Word, Photoshop, etc. felt like they loaded faster (however, I didn't actually time them).
I don't recall the exact system specs, but it was an i5 machine running Win 7 Home Premium with 6GB RAM and a 750GB Caviar Green 5400RPM drive.
The problem with anecdotal evidence like this is that I'm sure that no one is going to have the exact software/hardware setup I had. However, for the one test I ran, it cut the boot time slightly by having less fonts.
Just an idea:
If you need many fonts sometimes, then you'd better to have those fonts in other folder like c:\my1 and make a symlink of those inside files into c:\windows\fonts.
Everytime you close Photoshop (or other software you need many fonts for), then you can just rename that folder to to C:\my2 (so, all those fonts will be delinked automatically from c:\windows\fonts). And, whenever in the future you want to include all those fonts again (before you open Photoshop or etc), then rename the folder back to c:\my1,so the fonts will be resolved again.