This setting controls how much memory the SMTP server uses to store a message before it creates files in Section 8.5.3. If you have enough memory in your server then you might consider setting this variable to a value that allows the MTA to store your average message size or more in memory
If the MTA does not need to rewrite backward pointing addresses then you can set USE_REVERSE_DATABASE=0 in option.dat. I have in the past set this parameter while trying to get every last milisecond of performance out of an MTA. Most ISPs will not require the MTAs to rewrite backward pointing addresses.