Lag when calculating sales tax on a cart containing mixed tax rate items

[sucralose]

When items are scanned into the POS, there is a delay in calculating the total correctly if the cart contains items with mixed tax rates. The system will initially calculate with the highest tax rate of the items. For example.

first item: $10 will scan in and total at $11.30 (correct, because tax rate for item is 13%)
second item: $10 will scan and now cart will be $22.60 (incorrect, because second item has only 5% tax rate).

Then there is a delay (sometimes up to 8-10 seconds) before Shopify "catches up" and recalculates the total to $21.80 (which is correct when item one has 13% rate and item two has 5% rate).

This has caused us to overcharge customers, as we start typing the total we see on the screen into our debit terminal BEFORE the cart eventually recalculates to the correct total based on the mixed tax rates.

Would love for this issue to be solved, we just charged a customer $8.50 too much on her credit card because the tax calculation hadn't caught up.