MYOB Advanced API constant timeouts

I am using REST api to upload stock items, currently uploaded 250k items, but since 200k mark I have been constantly getting API request timeouts, so I added a retry mechanism which now results very slow upload, eg. 10 seconds for 1 successful upload. (Used to be much faster before timeouts)

How do I know if it's because of internal thorttling or due to large amount of records in database? Is there an offical support channel from MYOB?