iPad lagging when calculating form fields

I've created a form on acrobat, which builds a personalised customer quotation based on selected quantities for various products. (The quantity is multiplied by the price per unit, this creates a line price, all the line prices are added up in a 'grand total' box at the end). On a computer this form works fine, and the totals are constantly updating whenever you change a quantity in the form. 

However when we open this form on the iPad in adobe fill and sign, the totals are not reliable in adding up, sometimes the total stay the same, sometimes there will be a delay in the calculation, so we can never be sure that the 'grand total' is the correct price. Is there a solution to this problem? Thanks a lot for your time