Slow INSERT as compared to UPDATE

Hi All,

When I insert or update a record in a table in PostgreSQL, the time taken by
insert query is much larger (300 ms) than the time taken by update query (20
ms). If I think logically, there should not be a significant difference. To
insert a record, it will check/insert the entry in the index and insert the
record in the table (unclustered B Tree indexing on primary key). For
update, it will search the entry in the indexing and update the record. In
both the cases, the number of operations/disk accesses are nearly equal.
Then why such a big execution time difference?

Thanks,
Preetika