Pivot table calculated Field function slow

Dear All,


For example, I have a pivot table below

Product	Market	snapshot	WK1	WK2	WK3	Wk4	WK5
A	UK	Week1	30	30	60	80	90
A	UK	Week2	40	60	50	70	30
B	US	Week1	20	20	20	20	20
B	US	Week2	10	10	10	10	10

I try to use Pivot table calculated Field function to find out difference between week 1 & week2. However, it is very slow. Any solution I can be faster.
Thanks for your help.