You can usually ignore the delay functions and a great many LCDs will work perfectly. I have found some of the cheaper brands need extra time to get the job done and this is where the choice of a data and a command delay are useful.

When the RS (Register Select) line is 0 you are writing an INSTRUCTION to the LCD which probably will not be displayed but will cause some internal action such as cursor positioning. This can take a relatively long time.
DEFINE LCD_COMMANDUS 1000 ' SET COMMAND DELAY TIME IN us.

When RS is 1 you are writing DATA to be displayed. Typically this is faster.
DEFINE LCD_DATAUS 40 ' SET DATA DELAY TIME IN us.

My suggestion is to comment out these two defines and only invoke them if you have a display problem.

HTH
BrianT