900 seconds is 15 minutes - this is the timeout for Odoo.sh to ensure one User (one process) doesn't tie up the resources of the server.
You can import around 50,000 records in that time - so your code might be inefficient, or there could be customization(s) in the database that slow down record creation. Automated Actions are the biggest cause here - some developers have disabled them prior to import and enabled them after the import.
If your process can't import in less than 15 minutes, you have two options:
- import less records
- switch to an on-premise deployment where you can increase the timeout