---- dataentry configoptions ----
type             : constant
name             : MAX_PROCESS_MESSAGE
purpose          : How many campaigns to process at the same time
allowed          : Any positive number
default          : 5
status_s         : 
category         : queue processing
----

Even if you have say 100s of campaigns queued, this setting will get phpList to work on only the first few (default 5) and only continue with the rest when the first few have been sent completely.