The CPUSPEED is used by the optimizer when modeling the SQL statement to help determine the lowest cost access plan. Some operations are very CPU intensive and may not be good choices for slower processors.
When setting up DB2, the CPUSPEED should be set to -1 so that the first time DB2 is started, it will run a program to calculate the CPUSPEED, then update the database manager configuration. Do not set the CPUSPEED to a specific value unless you are modeling what access plan the optimizer would choose on newer/faster CPUs.
The default diagnostic level is 3, which specifies that all errors and warnings should be written to the db2diag.log file. In most cases, this is sufficient and will not cause any significant performance impact.
Setting the DIAGLEVEL or NOTIFYLEVEL to 4 will also log informational messages that can help debug problems if they occur; however, this can contribute to a significant degradation in system performance.