Understanding the timeout.
On the server, in addition to your site, there are other sites. Server resources must be fairly distributed among all sites, so the time of continuous PHP operation program is limited.
Usually, it is 30 - 60 seconds. This means that after 60 seconds the plugin work will be forcibly terminated, and you will receive a message on the browser screen: "maximum execution time of 30 seconds exceeded" or "Internal Server Error 500", or just a "white screen".
How does it work in reality?
However, in practice, the plugin works for 3-5 minutes. This is because it is actively reading and writing data to the database, and interruptions are disabled when writing to the database.
You often have already received a timeout message, but the plugin is still running. In such cases, if you press the F5 key, a parallel process can start - two plugins will work simultaneously, i.e. items can be added twice. There will be duplicates.
Do not rush to press F5 or refresh the browser page, which is the same thing. First, make sure that the admin/uploads/errors and admin/uploads/report report files do not grow in size, and that the price list line number recorded in the sos file does not change over time.
This is evidence that the plugin actually timed out.