We need to generate a massive amount of transactions at the end of the month, say 20,000 Invoices and 20,000 Customer Payments that must be created in a couple of days max. What's the fastest way to create those transactions? CSV Import, SOAP, REST, Restlet? Open to ideas if anyone has done something similar. We have a Premium Tier instance with 1 SuiteCloud Plus License.

What is the source? Almost regardless of what you may evolve it into in the future, integration-wise, you should start with CSV, it is by far the easiest way I would say, unless source system has something like an adapter or similar turn-key option. With one suitecloud plus license you will have that running and not blocking other stuff (2 queues) - the real performance is likely more a result of total numebr of lines rather than transactions, but cant ever see that taking more than a day - in any case, I would strongly recommend going the CSV route till you have it nailed down process-wise, had some feedback etc.

I agree CSV. Or Celigo data loader

you want to start with the minor legwork of figuring out how long it takes to make one customer payment and one invoice using your chosen method, and then seeing if its good enough to make your transactions fast enough

Thanks all for the insight, will look into the options.

CSV Import will be slow as you can't use concurrency as each transaction consists of multiple lines. REST/SOAP is generally the fastest solution where you can uploadList, not 1 at a time and you can also utilize concurrency based of your NS account's limit. Another option that we have opted for for some use cases is to import CSVs into NS and then use a Map reduce script to create the transactions.

Thanks @Martin Horacek