Stated simply: many versions of syslogd will fsync() logfiles after writing a messages to them, in an attempt to make sure that the message makes it to disk in case something happens to the system immediately afterwards (crashes, loses power, etc). This obviously can have an impact (sometimes a significant one) on any other IO activity going on at the time.

On some but not all systems with this feature, you can this off for specific syslog files by sticking a '-' in front of them; this is especially handy for high volume, low importance log files, such as ones you're just using for statistical analysis. (For example, one system around here has a relatively active nameserver that syslogs every query. You can bet that we have fsync() turned off for that logfile, and when we accidentally didn't we noticed right away.)

(From the moderate amount of poking I've done, Solaris always does this and has no option to turn it off, FreeBSD only does this for kernel messages and can turn it off, and Linux's traditional syslog daemon always does this and can turn it off. I don't know about the new syslog daemon in Fedora. OpenBSD doesn't say anything in its manpages, but appears to always fsync().)

As a side note, if you really need syslog messages to be captured, I recommend also forwarding them to a remote syslog server. That way you have a much higher chance of capturing messages like 'inconsistency detected in /var, turning it read-only' (which has happened to us), and you have a certain amount of insurance against the clock on the machine going crazy.

(A central syslog server is also a convenient place to watch all of your systems at once and easily correlate events across them.)