Hey @derrick , thank you SO much for your replies and help. I think I figured out what we have been doing wrong - I think our generated UUIDs have been incorrect, which is why it hasn’t been working. Thank you for your code snippets and assistance with this.
I’m going to do a lot of further testing and get our bulkCartUpdate working, and then I will reply back here with our findings about how this impacts speed and performance.
@Victor_Chu - thankyou for your help too. I will report back here as soon as possible 
Hi everyone, just wanted to add my 2 cents to this thread.
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, approx. 4s in my experience. 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.
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https://github.com/Shopify/ui-extensions/issues/1671
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Some delay we observed (admittedly this was over a year ago on the dev environment so not sure how relevant these numbers are anymore):
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This was also without a cart transform as they weren’t available at the time.
I’ll get some better numbers on the timings for the bulkCartUpdate and post them here shortly.
Okay some tests I did just now, these were in the dev environment so probably ideal, real world numbers would likely be greater than this.
It seems to take around 2.5s, these were with a varying number of line items and a cart transform.
It’s definitely an improvement over the addLineItem method which increased the delay proportional to the number of items added but I still get merchants complaining about the delay quite often, probably the ones with poorer quality internet connections.
addLineItem appears to take a similar amount of time.
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.
Hey @simon_b ,
Thank you very much for adding your feedback here and also for sharing your logs, it’s extremely helpful.
I wanted to let you know that I also opened a second issue here, which is more focused on the shopify.cart.addLineItem function which, in our further testing, seems to be where the main bottleneck is.
If you have time, it would be great if you could read through that thread and add your feedback there also
We also see similar feedback from merchants with poorer internet connections - it seems to be a common thread. I’m hoping that the Shopify POS team can address this and provide a way for the functions and api to perform well regardless of internet connectivity.
