When you say that your "uptime" average goes over 2.4, you are not really determining what the problem is.
Best to run "vmstat 5" to see whether the processes are CPU-bound, IO-bound, or whatnot. The vmstat man page has more info on what the numbers mean.
If the actual CPU usage is generally low, then the answer is simply to get a faster disk. You can mount /var/spool/mail on the faster disk and separate the disk operations for email from those for the web pages and database.
