[royo]

Well, clicking a thread for example will make a request to the MySQL database to retrieve the post data contained inside that thread. The more requests that happen per second, the slower the server will be. You could have a 4 x quad opteron setup with 16GB of RAM, but with a simple SATA drive, the 'wait' time of the HDD will pretty much render all the other hardware useless.

Something that should be analysed is the %wa that appears when you do a top command in Linux; if it is over 10% you will start to see slowness.

A SCSI drive spins faster than a normal IDE or SATA drive, decreasing access times and allowing for more mysql interaction per second. You should consult with your service provider about this, as they should be able to tell you if this is causing trouble during peak load times on your website.