Hi everyone,
Adding some more info here from my own testing (originally posted here).
We have been using the bulkCartUpdate method for some time now along with a cart transform and whilst it does provide a more consistent delay, it still adds a decent amount of time to the operation. Which in some contexts is undesirable, some merchants are not tolerant of this amount of delay.
I did some testing some time ago which I provided in an issue on the Shopify/ui-extensions repo, looks like issues have since been disabled so I can’t reference it anymore but here are the results I had:
-
Some delay we observed (admittedly this was over a year ago on the dev environment so not sure how relevant these numbers are anymore):
-
-
This was also without a cart transform as they weren’t available at the time.
Numbers for the bulkCartUpdate I got today:
It seems to take around 2.5s, these were with a varying number of line items and a cart transform.
addLineItem (also from today) appears to take a similar amount of time:
Screenshot 2026-05-13 at 09.00.29710×220 11.3 KB
Ignore the 5s+ one that one was out of stock 
If nothing has changed since my initial tests over a year ago then it looks like the cart transform adds another second to the latency.