LIMIT 20 OFFSET 4980 does not jump anywhere. The server produces rows in order, discards the first 4980, and returns the next 20. The cost is O(offset + limit) — page 250 costs 250 times page 1.

An index on the sort column removes the sort, not the skip. The rows arrive already ordered, and are then still produced and thrown away one at a time.

page   1   →  20 rows read,  20 returned
page  10   → 200 rows read,  20 returned
page 250   → 5000 rows read, 20 returned

The number that matters is the whole crawl, because that is what a background export or an infinite-scrolling client actually performs: 250 pages costs 627,500 row reads by OFFSET and 5,000 by cursor.