I’d recommend that you instead use one of the existing import scripts and modify it for your needs. It will be 10-100 orders of magnitude faster. I wrote several importers before I learned Ruby.
You can find out about rate limits here: discourse/discourse_defaults.conf at 1f34f653bfb6ccb39a4f4f8085b07e9d94389e6f · discourse/discourse · GitHub and here: Available settings for global rate limits and throttling