Performing a GL Writeback of a large number of transactions (1000+) can take a longer-than-expected time.
The upload time for a GL Writeback increases non-linearly with the number of transactions in the worksheet:
The root cause of this is a performance issue in Acumatica, apparently related to subaccount validation. When data is coming from a source other than Acumatica's Journal Transactions screen, Acumatica appears to perform subaccount defaulting (something that - within Acumatica - is already done while on that Journal Transactions screen). If the number of transactions is small, the issue is not observed, but for lists of more than 1000 transactions, it can take a very long time.
Acumatica has acknowledged the issue (case 176646) and will improve performance in a future update of Acumatica. In the meantime, a hotfix that significantly improves the upload performance can be published on your website.
The only caveat is that after publishing the customization, you will have to always explicitly fill the Subaccount column in the GL writeback template (e.g. using the default subaccount such as 000-000 if you do not otherwise use subaccounts).
The reason for this is that this customization turns off the computationally-expensive subaccount defaulting logic in Acumatica's Journal Transactions screen for external tools such as Velixo (workflows happening directly from the Journal Transactions screen will not be affected)
How to Import and Publish a customization project
For information on how to import a customization project, see Import a Project in the Acumatica documentation.
For information on how to publish a customization project, see Publish a Project in the Acumatica documentation.
If you experience this issue and do not wish to install the Acumatica customization, as a workaround, you can use the Load Records from File functionality in the Journal Transactions screen in Acumatica: