PDF form auto populating is slow

I have a form that I am having an issue with.

I have 2 drop down menus which are 1 -5 they are added up in a 'hidden' box whin intern is read with a validation code resulting in a word depending on the number.  This works on another form perfectly but for some reason on this form it is slow and updates when you go on for the next field or two.  would this just be due to the quantity of boxes?  (the working one has 2 pages with a total of 12 setups the form I am having issues with is 9 pages with about 13 on 5 pages each)

Any help would be appriciated.

Thank you for any help that can be given

Matthew